My understanding is that there are two main conditions. The first is that the defense has a long suit with which they can set up a "ruffing finesse," usually against the declarer's trumps by leading through a newly created void (usually after a lead or two).
The second condition seems to be when the defense has a number of "intermediate" trumps, typically ranging from Q to 8. (If the defense had four trumps 5432, a promotion wouldn't work, and if the four defensive trumps were AKQJ, it wouldn't seem to matter.) With a promotion, a defender uses an intermediate trump to force out a higher trump to promote partner's intermediates.
Is this second precondition in fact the right one? And if so, how do you, a defender, detect the precondition either when making an opening lead (West) or responding to one (East)? Is it because you see a lot of low trumps in dummy and some intermediates in your own hand? And then do you infer that if partner has one or more trumps that they are likely to be intermediates?