I understand what you are saying. I just never tried to analyze the mechanics of the motion. However, let me give it a try: In general, the point of contact will always lie at a “higher location” on your fingernail than the point of release (refer to figures A and B). The terms “higher” and “lower” are relative to the distance from the soundboard. Figure A illustrates a thumb posture that places the point of contact at the middle of the thumb nail. The point of release should occur at some location below (to comply with the desire that the string is pushed toward the soundboard as the string is plucked). Figure B illustrates a thumb posture that will place the point of contact on the left edge and the point of release at the middle of the thumb nail.
So contrary to your second post, I believe the relationship between the points of contact and release are conceptually the same regardless of whether the point of contact is at the middle or the left side. In either case, the thumb nail should pushes the string toward the sound board and thus the point of release must be located lower than the point of contact. The main differences between the two situations are (1) orientation of the thumb and (2) the trajectory of the thumb stroke (not shown in my drawings).
Hope the above helps -- Rocky