The Pendulum Swing
One of the most common mistakes I see from a technical standpoint is people using a "pendulum swing" rather than a flat, level swing. The pendulum is a swing that relies on absolutely perfect timing. If you're a little early on your swing, you skip the ball due to the angle of your racquet. If you're a little late, the ball goes up. Your timing has to be perfect in order for you to hit the ball straight. There is no room for error.
Well, as a high level player and an instructor, I can tell you that when you're playing against another high level player, you want to maximize your room for error. It's very difficult to be perfect against a good player. By using the flat, level swing that I teach (and most of the professional players use), you will give yourself more room for error, increase your power, and increase your accuracy.



In the above examples I'm using a flat, level stroke. Let's assume I'm trying to hit a shot straight in to the front wall. In the first example, I'm too early and my racquet angle is too far forward. So what happens? I hit a cross court shot. Not too bad.
In the second example my timing is perfect and the ball is hit straight in.
In the third example I'm too late. What happens? I hit a splat shot. Again, not a terrible thing.
So with a flat, level swing, whether you're late, right on, or early you're going to hit an effective shot, and keep the ball in play. With a pendulum swing it's either a skip (if you're late and racquet angle is down toward the floor), a good shot (if you're timing is perfect and your racquet is angled perpendicular to the floor), or way too high (if you're early, and the racquet angle is up). There are a million other reasons to use a level swing (deception, power, accuracy, consistency, less energy, etc).
Here are 3 video examples. The first is the dreaded pendulum swing (please don't use this!) by someone from youtube. And the others are videos of myself, using a flat, level swing.