"Leading language" is almost a dirty word due to its connections with false memories and iatrogenic cases of Dissociative Identity Disorder. And indeed, people do really nasty things with leading language without ever realizing what they're doing. In response to that, the Clean Language people harp on the value of deliberately refraining from injecting one's own assumptions into things. They have a lot of very very good points. For example, if you hear "I feel strange" your automatic response should be "what kind of strange?". If instead, you say something like "do you have a headache?", then you're fucking up and giving people unnecessary headaches.
However, there is a very very good use for leading language too. It's an essential part of the effective use of language. "Leading language" basically means "any language that refers to one's own frame". In other words "language that assumes it matters what you think". If you're not a worthless fool, sometimes it does.
Double binds (in the Ericksonian/nlp sense, not the Bateson sense) make a good example of this. The classic salesman example of "would you like to pay cash or credit?" frames it like a purposely trying to hoodwink the customer into forgetting that not buying is an option - as if they're doing the "false memory" failure mode on purpose because they're a greasy no good salesman. Can be, but doesn't have to be. For example, sometimes when I'm feeling cocky and obnoxious, I'll offer the double bind "would you like to realize I'm right now and skip straight to learning what you're missing, or would you like to argue about it first and end up feeling really dumb for trying to avoid the inevitable?". In this case, I'm purposely presupposing my rightness so hard that it can't be missed. I'm taking the difference in our frames and highlighting (not hiding) it so as to prevent any "he's trying to fool me" impulse. The possibility of responding "I'd like to argue it and come out right, please" is highlighted, and the whole reason that double bind works (in the cases in which I use it) is because they already know that ain't gonna happen.
The over-the-top-ness of that example kinda functions as a bit of implicitly clean language in that it reminds them of the choice to not enter my frame, but the language itself is very very leading. It only makes sense within my own frame that leads to the outcomes I want. A more "neutral" example, which neither highlights nor hides potential frame challenges, would be the in-control parent who offers their kids a choice between going to bed at 8:30 or 9. The possibility of frame challenging is still there, but if the parent is actually in control, there's no need to highlight or hide it, because the parents frame is just the reality.
Basically, any time what you think matters, leading language is going to be necessary. Stating "I am in control here and you cannot choose to go to bed later, even if you want" doesn't need to be said if it's not actually up for debate (and if it's up for debate, then making a bold unqualified claim there isn't a great way to start!). Even if they yield that explicitly, you still need to ask if they want to go to bed at 8:30 or 9 - if you don't want to try to ask cleanly "would you like to go to sleep again? If so, when?" and then tell them "no" if they say anything later than 9. Using leading language correctly, within-frame, boils down to not-being-a-bitch when it comes to asserting your frame. Don't be a bitch by trying to not give them a chance to say they don't want to buy, but also don't be a bitch by refusing to acknowledge that they're sold and asking out of insecurity "are you sure? you don't have to buy, you know".
Keeping yourself from using leading language accidentally is just a matter of tracking what you know and what you don't know. Sometimes this seems like it ought to be quite obvious (Do you have any reason to believe that the "feel strange" means headache? Then what the heck are you doing pointing at it!?), but in general, it is not. The problem is that within their frame, they're using leading language correctly. It's the frame itself which is wrong and overconfident, which is a much bigger and more general problem. It's one thing to tell therapists not to lead clients to believe they were molested as a kid (again, seems obvious from the outside), but if their therapeutic theories predict the abuse will be there, then "don't use leading language" comes off like it's just overly-cautious advice, such as always wear your seat belt, even if you're not going very far. Like, "yeah, you're right, you're right. It's a good idea [but who are we kidding, it's no big deal, I'm just driving down the street and this person was definitely molested as a kid]".
But even that undersells the difficulty of avoiding accidental misuse. If a kid burns his hand on a fire poker, it's a pretty natural response to wince at his pain and offer to get him some ice so it doesn't hurt. It even seems like an empathetic and nice thing to do, right? Unless you've already started to think like a hypnotist, it probably never even crossed your mind that it doesn't have to hurt. Not-hurting just isn't something you can comprehend and feels impossible, so anything you do is going to presuppose that it's gonna hurt.
Except you'd be wrong, and you'd be unintentionally enforcing your expectations of suffering onto the kid who would have otherwise been fine. My one word pain cure would not have even been necessary if it weren't for everyone else not realizing what they don't know and accidentally using leading language that they were completely blind to. Merely convincing them not to use leading language wouldn't have changed a thing. The reason my response was as effective as it was was that there never was a real problem in the first place, and my response was clean enough for him to see that. (It's worth noting that in this case, my response of "hurts?" does presuppose that pain could be the issue, but I only said it that way because of direct observations that he was in pain, and my focus wasn't on "pain is an issue!" but "is pain an issue?", giving him a way out. It'd be like hearing "I feel weird" and noting that they look like they have a headache and then instead of saying "what kind of weird?" saying "it looks like you're feeling it in your head?", and drawing attention to the fact that you aren't sure, might be wrong, and certainly are not to be followed to that conclusion.)
To make sure you don't accidentally misuse leading language, you have to maintain a very strict openness to things you don't understand (pretty damn important in general, too). In particular, before presupposing that a burn is going to hurt, ask yourself "how do you know it hurts? What if it doesn't?". If your response is a sort of indignance as if to say "You can't question that! burns cause pain and pain is bad! everyone knows that!", then you fail. Success doesn't look like "won't imagine" or "afraid to imagine and expect not to find anything". Success looks like "don't need to imagine". Like, "Because they told me it hurts. You can see it in their face, look at the tension in their eyes and the corner of their lips turning down", or like "Do you think it doesn't?" with an openness to hear that they don't. Of course, it's too tedious to consciously ask these questions every time you do anything, but over time you can develop the habit of not overextending your frame so that if those questions were to be asked, you'd have an answer.
That's a quite big and hard to meet requirement, which makes clean language a big deal. It's a super common failure mode, especially when working with highly suggestible people, to suggest their problems away, only for them to come back out in the real world since it wasn't properly dealt with - you just put a "it's not a problem" interpretation there over the still existing "it is a problem" interpretation - instead of collapsing the problem interpretation and leaving only one.
If you have that handled though - if you're not flinching away from seeing what you don't see, and you're not flinching away from frame challenges - then use your leading language without shame when you have a viewpoint to share. Have a product that really is so amazing that it would be a mistake to not buy it? Then go ahead and lead them right up to "cash or credit?" if they'll follow - and back up and get back onto the same page when they don't.
If you actually got it handled so that you know it matters what you think, then lead them to it. And when it matters what they think, use clean language to make sure they're on the same page, and didn't just get sucked along for the ride. I'll often do something like "It can be totally fun and non-scary. You can see that, can't you?". Followed by "Is it dangerous? For real?" and "But what if I'm wrong?" until I can see that it's taken enough root to be stable on its own having superseded the old frame instead of just pasting over it.
Use leading questions when you want to lead someone to a certain frame of mind. Use clean language when you want to find out where they truly are - or want them to realize for themselves.