My understanding of the dropper seat post, from years ago when I looked into it briefly to see if I needed or wanted one, was that it is released from full set extension by a control, say a button on the handlebars operating a cable, in the imminent approach of a possibly hurtful incident*, and drops down into the seat tube, thus making it easier to jump from the bike or at least to fall clear of it.
* Motor racers don't like the word "accident", implying as it does an avoidable error of judgement or reaction, so I presume extreme mountain bikers don't either. The use of "incident" instead implies an error by someone else or, since they could still be in good enough fettle to blame you, best of all an Act of God.
Whenever I've thought about the dropper post again, it was because the long tail of my foul-weather cycling jacket caught under my Brooks saddle in voluntary dismounts, in which case the dropper release would still be under my thumb. But that hasn't happened often enough to inspire me to source and buy the thing.
