The forehand loop off underspin marks the transition between the defensive part of the point in which underspin is used to control placement and the aggressive part of the point where the player capitalizes on a weakly played underspin ball and starts the attack.
This stroke overpowers the spin on the ping pong ball and produces strong top spin. Compared to a loop off top sign, this stroke starts lower and requires a strong upward component.
It also is usually taken a later in the ball's trajectory. The more aggressive the shot, the earlier the timing. When possible, some players take it even before the highest point.
Come in closer to the table tennis table and drop your hand further down than you would for a top spin ball. Your ping pong paddle is more open and your knees are bent more than for a shot off top spin.
Push off with your right leg, swing up and forward with a quick powerful snap. Your whole body must move forward. This is the contact point.
Your weight shifts forward onto your left leg while you follow through strongly.
Make sure that you drive the ball forward enough to clear the net. It is a common mistake to focus so much on lifting the ball that we forget to make sure that the ball reaches its maximum height over the net and not before.