Hitting a looping topspin forehand with a high arc over the net is actually a very aggressive shot. Used properly it can be a very high percentage play. Most amateurs believe that to emulate the pros, they must hit the ball low and flat over the net. This is not true. By imparting topspin on the ball, and hitting it high, you are making the ball bounce far over your opponent’s head, and it is difficult for them to deal with. It is the most effective shot against people who play with a continental or eastern grip.
Here’s a gif of the regular height over net during a professional rally.
The lines are faint, but you can see a yellow, green, and red line just above the net. The peak of the green line is a typical rally ball, with lots of height over the net. The yellow line peak is sub-optimal as you will not get as much depth on your shot. The red line is only entered when hitting an attacking ball from the short-court and you are in full control of the ball.
Here, I am illustrating one key that will help you hit a looping topspin forehand. The green arrow traces the path that my left hand takes. If you look carefully, you can see that my shoulders turn normally just like any other shot, but I move my left hand in a high arc over my head before initiating my swing. By rotating my body with my left hand up high, it forces me to drop my right shoulder, and get the racquet well below the ball, which then causes a higher trajectory shot.
Also notice that I still accelerate at the ball, and rotate normally with my legs and hips.
In contrast, notice how in the below video I bring my left hand low and straight across my body as I rotate, and the ball flies low and flat. The ball will tend to follow the trajectory you trace out with that left hand.