I’m sorry this is irrelevant to this post, but I was googling and I found one of your old posts (4/12/06). I just wanted to tell you that I have this same apparently very rare problem of not being able to bend my thumb in on knife-hand techniques. Several years ago I realized that my index finger is required to follow my thumb’s bending at the distal joint.
I never really looked into it; I suppose there’s nothing really to be done for it, but it doesn’t interfere too extremely with the technique as long as you bend your whole thumb in toward your palm. It doesn’t look nearly as cool though!