The primary reason for over gripping is to eliminate a severly bent wrist when riding. If you grip in a natural position, your elbow will drop down far, and your wrist will be bent at almost a 45 degree angle. It is completely un-natural, and can cause severe problems when hitting jumps, chops, or railing corners.
I got in the habit years ago on a 2smoke, and had a hard time getting rid of it with my 450. The main thing is to keep your hand moving on the throttle. If you grip, and go, you will find yourself in siuations such as where you are now (hope everything heals fast and proper by the way). It took me wahile to get used to to it, but once it is figured out, it really is a easier, safer way to ride. Just remember to try and keep your wrist and arm in a slightly neutral spot through all areas of the track.