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Make sure your abs stay tightened and your back is straight. You should raise up until your upper body is completely vertical. You should slowly lower your body until the back of your shoulders touch the bench. Keep your back straight and your abs tight. Try to do 10 repetitions of this exercise per set. Repeat until you've completed 3 sets.

Summary:
Raise yourself from the bench by bending your waist and hips. Lower yourself down slowly. Repeat for the desired number of repetitions.