Mastering the smash when you're starting out is not easy... We often have a tendency, for some mysterious reason, to pistachio the ball into the net...
A very good workout is to do smashes targeting areas.

This exercise is on the one hand technically very productive, but in addition it allows players to visualize an area and then hit the ball... visualizing the area (mentally) allows the body and arm to be conditioned to seek out this area. (it's stupid to say but clearly visualizing an area will prevent us from putting the ball in the net)

For the exercise it's quite simple, the coach sends to lob and he designates an area: the player Smash in the targeted area.
The player can also decide and designate the area himself.

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