Which sentence is correct? "I slap my hands at his wrist." and "I slap my hands on his wrist."
Or they are both correct? "at" indicates the direction of target and "on" indicates the side of target?

Also does "hands" indicates that I use two hands? But normally we use one hand to slap... rather than two. In this case, should I say I slap one hand at his wrist?

Thanks for your answer.

Question

Which sentence is correct? "I slap my hands at his wrist." and "I slap my hands on his wrist."
Or they are both correct? "at" indicates the direction of target and "on" indicates the side of target?

Also does "hands" indicates that I use two hands? But normally we use one hand to slap... rather than two. In this case, should I say I slap one hand at his wrist?

Thanks for your answer.

Answer

The teacher slapped the student's wrist. 
I slapped this person on the wrist.

It is much better without the word "hands" and 'at'. It is (already) understood that you used your hands, or a strap/a ruler to slap.