...is the last one
This device you refer to is designed to invert you "fists" as you press away from your body. It is a Neuro pattern much as a punch is done properly.
If you start you punch from your side with you fist with the thumb up and push out and up to about nose height while rotation your fist(palm down) then that is the motion of the perfect push up.
This devise is two separate pieces and hand placement can be as mentioned above to accent different areas of the muscles used in the push up.
Not a waste of money, but not necessary. Blitzzz