Biomechanically, the Forehand Underspin Push is one of the hardest to learn.
The coordination must be very precise and timing is one the most important elements of this technique.

The motion preparation is the key and once you are in position, the execution is simple.
we are going to break down in 3 steps.

Preparation
From the ready stance, approach the ball following the steps below.
1 – Move your foot towards the ball with a big step, placing the foot underneath the table and bend your knee.
2 – Extent your arm, place the paddle close to the ball and make sure you keep your muscles engaged.
3 – Open your opposite arm to balance your body. 
All 3 steps must be synchronized

Execution
Once you are in position.
1 – Swing whole body bending your knee even lower towards the ball.
2 – Push the ball right off the bounce
Both steps also must be synchronized.

Watch the Video