The primary muscles worked during this exercise are the mid shoulders (delts), secondary triceps.

Adjust the bench to almost 90 degree incline if you prefer to use a back support for improved stability.

Adjust the pressing arms for the desired range of motion. Lowering the handles down to your chin will give a greater range of motion, but will also increase stress on your shoulder joints. Make sure you don't lower the handles so much that it hurts in your shoulders.

Execution

With an explosive but controlled motion push the handles over your head. Slowly lower the handles. Repeat this motion until you have completed your set.

Keep your back straight during the exercise and do not lock your elbows at the top of the movement.