I think what they are all tying to say is : Your horse needs to be very forwards before you attempt piaffe. For my horse(self trained) I apply rein- back aids(from seat,weight aids) when there is sufficient forwards movement(umbrellas do the trick as well as pigs,tractors,local hunt etc.)Once you have achieved enough collection,maintain it with your seat(rhythm) and leg(impulsion)
Only ask for a few steps. Its very hard work for them and if you get the balance wrong ie too much hand,you might not get the desired result!
I have found the hunt to be an excellent aid to non-stop piaffe on an older horse.
With youngsters I've had to resort to lorries and/or vans towing trailers full of crashing metal passing the school.
I have heard that umbrellas and prams can also be highly effective if used correctly.
S