1. Hitting off a closed stance as against a running stance (an open stance that has become a sideways elevated lunging motion during the hit). Especially on the forehand side. A lot of players like to hit off a closed stance when hitting a backhand on the run but a power move will have to come into play when really pushed wide.
2. Jumping up on contact. Because you are a long way off court the tendency is to try and gain more power and/or try and create more spin and players try and jump up to do this. It is much better to drop the weight down and keep the angles in the legs than lift the head and lose control of your body.
Losing the angles in the legs means the player comes up with their legs as they hit the ball. The legs straighten up and all balance and athletic loading of the muscles is lost .A really good way to fix this is to have the player able to see the ball that they have just hit bounce under the tape of the net as they are looking through the net. Or wear an athletic training belt that has a bungee cord attaches to the waist and pull down to stop the player jumping. It is important to snap the hips not jump up.
3. Not keeping the steps even when running to the ball. It is like run up to a long jump. Lots of little steps at the end will throw off the timing and balance on the hit. Running for the power move can be practised up stairs and then mimicking the power move when on the flat section at the top of the stairs. This will help emphasise good even steps. Though, 2 stairs at a time is more realistic then 1 at a time as steps to the ball need to be strong and powerful.
a. Feel like you are scissor kicking the legs when hitting. This helps with the concept of feeling the kick back balance move as against the leg curl balance move.
b. Teach the move with jogging around the court with good even steps and rhythm and just swinging the racket while nice and relaxed. When you do this it will not feel natural to hit off a closed stance as hitting closed breaks the stride rhythm.
c. Drop lower as you hit and most of the time you should think that you intend to recover i.e. that you intend to use brake steps. This will automatically keep you down on the shot.
d. Extend through the swing don’t flick the wrist at the ball.
e. I also like to teach the racket finishing around the shoulder of the opposite side of the body as against finishing behind the head on the same side of the body. Let the professional players hit this shot. I believe the Buggy Whip (this is what I call this swing) puts a lot of stain on the shoulder and makes you contact the ball late.