HOW: Get set up on your knees with your feet/ankles anchored. Be kneeling tall with your hands supported on a swissball. Perform the exercise by rolling the ball forward while keeping your hips extended, your trunk/spine relatively neutral, and slowly lowering yourself using your hamstrings. Roll the ball as far forward as you can, then return to starting position and repeat.
FEEL: You should really feel and focus on feeling your hamstrings working the entire time. The swissball makes it easier for your shoulders and your core to help out, but think about maintaining tension in your hamstrings and pull 'up' with your feet the entire time.
COMPENSATION: Do not lose tension in your hamstrings, avoid folding at your hips - keep them pushed forward. Focus on maintaining a relative neutral trunk/spine