To me it's simple, once the receiver has the ball in both hands and two feet down in bounds (or a knee, elbow, hip, etc...) he has possession. After that if the ball pops out, it's a fumble. If he crosses the plane of the goal line, it's a touchdown. Forget all the 'football move' and did he 'complete the catch' bs and keep it simple.
But as it stands, i don't know why receivers aren't more conscious of holding the ball all the way through.