In the world of taijutsu, legs are like pistons. You let the pressure out of them to lower yourself to the ground, and increase the pressure to lift yourself up. Take Morote Gari. You drop to a sprinters stance - releasing the pressure in your knees (performed by the calves and thighs) and position yourself at the knees of uke, then increase the pressure in your knees to take uke to the mat.
Hokojutsu is the same thing. The key to moving my bulk around silently is to not place my leg, but lower it, and then lower my weight onto it. If I just place it, I thump. If I lower it, I don't. It's a totally different way of thinking.
There are a bunch of examples. When I perform Oni Kudaki, for example (in the classical way) I slide into uke with my back foot and lower by center of gravity on my pistons to take balance. Then I raise myself after I get the lock on the elbow. The legs work just like hydraulics to lower and raise my body.
It's surprising how much preparation is a part of hensojutsu. I have been reading On Acting by Sanford Meisner, and his approach to preparation has me thinking a lot about hensojutsu and reality. Meisner says that 'acting is living truthfully under imaginary circumstances.' Admittedly, if you have one hell of an imagination you can just jump up and do what is required under the circumstances.
So, Meisner has his students prepare. The difference is, he has them prepare for the emotion not the action. For instance, say you have a scene where your sister has died. But you are an only child. You don't know what it would feel like to lose a sibling. Well, let's get close, Meisner says. Ever lose a parent? Dear friend? Was it pretty miserable? Alright, THINK about that and say the lines.
For my 6th kyu test I have to perform hensoijutsu as Yado Nashi, or a homeless man. I originally thought to do research, and that is a good idea for the facts - where I live, how I got there, etc. For my actual preparation, though, I have to key in on emotion - hopelessness, or determination depending on my character. I have to call up something that harbors the emotion of the character as well as remember the facts. It isn't like the facts will be more true with the right emotion, but they sure will be more believable.