Underrotating can also be handle location. Make sure to pin the handle to your waist. This will speed your rotation which can help if you aren't getting a lot of height.
Also, a coach in Orlando told me to think of the this trick as a 3-speed transmission. 1st gear is beginning to drift back to wake, 2nd is to progressive with a harder edge, and 3rd gear is to pour on the speed. You should be in 3rd gear at the trough of wake.
If you do this, you will not underrotate. You have to go for it and stay on that edge. With enough rotation and speed you can do this w/ your eyes closed!