If you tend to flip at the ball with your hands at impact. You need to practice these drills.
Flippers have to rely on timing to make their shots go straight - the timing of the hand action through impact determines the outcome of each shot. And when impact between ball and face lasts for approximately 1/2000 of a second it's not that easy to be consistent - especially under pressure! The better you play, the greater your anxiety level, the less you control the timing of the flip - not a good recipe for low scores when it counts.

Watch the following video to get a sense of what to feel at impact.