The backhand loop off underspin has almost become as common as the one from the forehand. Being equally comfortable to open the attack from either wing, multiplies your options and reduces the demands on your footwork.
The adjustment between looping a top spin ball and an underspin ball is similar to the one required for the forehand. The timing is usually a little later and it requires you to bend your legs a bit more. Since the ball has a tendency to drop, it requires more upward force.
Before the ball bounces, step back a little with your left foot so your feet are almost parallel to the in line. Shift your weight a little to the left and twist your body slightly. Drop your right shoulder and pull the racket back.
After the ball bounces, turn your wrist backward and start the forward motion by pushing your elbow forward and upward. Now turn your body into the ball and let your hand catch up with your upper arm while straightening your wrist. Contact the ball after it's started dropping just in front of you. This is the contact point. Follow through high and then recover. This shot depends a lot of full commitment. You must snap hard on contact and make sure you let the ball dig into the table tennis rubber so it picks up enough spin. Push your hips forward to help the momentum.
I saw him push to my backhand, I try to bend my knees. I try to stay down to keep my back down and then it's important also that I get good wrist snap with my backhand. With this wrist power, it make rotation, it make you top spin heavy.
Start practicing the stroke against medium underspin balls first so you can perform it a little slower. Then as [your partner 00:02:13] to increase the underspin and rip through the ball.