I don't know what the trick is based on - weight shifting or the fact that you become limp and simply slide out of the person's arms.
I don't particularly believe in it though because I saw a high-ranking person demonstrate it and it didn't work.
I look at the ki tricks with a somewhat similar perspective. I was fed a lot of stuff about the unbendable arm and flowing ki through it. Looking at the mechanics of the situation and how muscles work the answer is really much more mundane--if muscles don't work against one another, they are much stronger. It just so happens that the visualization does help produce the desired result.