Just because your objection parallels my comment in form doesn't automatically make its content a correct refutation; and someone other than me has warned you that the tactic doesn't serve you particularly well.

I'm aware of how people get angry when their own argument methods are turned around and force them to think critically about the basis for their own beliefs -- though I don't think that's happening in your case. (The anger on your part isn't happening, I mean -- I do believe you are reflecting critically on your own beliefs, or at least are making a genuine effort.)

The point of me mimicking your form was not to be cute (although that was a neat side effect), but rather, to show that a simple reframing of the issue -- by highlighting different salient aspects -- would reverse the "obvious" answer to your question.

On one hand we have advice that is about body posture, and on the other hand we have advice that is about persuading yourself of things that are not true, such as thinking of an adult human as if they were a child.

You claim advice about body posture to be benign, while believing false, offensive things is obviously bad by comparison. (The latter is a strawman of course: the advice is to, like an actor, go into a different mindset in order to have a generating function for your actions, which turns out to be preferable by the "target" of it. The advice is not to believe that adult women are disobedient children as if it were some more objective or universal aspect of reality.)

Do you or do you not agree that "think of her as a child" involves changing your mental state, while "show cleavage and arch your back" does not?

Of course I agree, but this is a poor metric. Isn't it more important what the advice causes in the other party's mind? If "think of her as a child" generates actions, on my part, that the woman deems preferable, what does it matter that my mental state is changed? If a woman uses attire and posture that causes me to "think below the waist", isn't the impact on my mental state more important -- because of the diminishing of informed consent [1] -- than the impact on the woman's mental state?

Your reply above directs attention away from this difference and toward the supposed "history of success" of the first form of advice.

Because, as explained above, it's not apparent how that's a relevant metric or difference.

This is shifting the goalposts, if your intent is still to understand why the first form is more often objected to. Whether the advice is sound or not is a separate matter.

If the advice actually benefits women, that should negate any objectionability of the advice that is grounded on harm to women. Failure to speak frankly about the commonality of the kind of woman benefitting, while instead giving full weight to the supposedly-universal preferences of the most vocal feminists ... to me, that looks like a social failing.

[1] Yes, yes, I lose status by mentioning that this can happen, &c. C'est la vie.

someone other than me has warned you that the tactic doesn't serve you particularly well.

It isn't usually a successful tactic, which is somewhat of a shame, given that it can serve to demonstrate how a particular (mis)use of argument is flawed. People on average don't have the respect for consistency that I would prefer.