Understanding Tennis

on the upper incoming ball it is too low for an overhead — it is still a volley

on the serve one of the choices is to platform the stance

dip the racquet head to the ground to increase your upward swing

you need to get  your back knee below the incoming flight path

bringing the ball into a better hit zone 

watch Denis get a big coil start on his backhand 

if you jump stay core balanced

like all your shots– see the hit

move in on the angle to cut off the run of the shot 

on the serve you go up and out on the hit