Here is Gustavo Kuerten hitting a running/buggy whip forehand off of a sampras first serve. He's uses an extreme grip. He has a great running forehand.
I think the running forehand is one of the easiest shots to learn. Most people I know that have good ones, just learned the shot on their own without formal instruction. A lot of situations where you are late to the ball, or you are trying to hit an extreme angle, just kind of force you to learn how to hit a running forehand.
The way I think of it is: I'm using my opponent's pace that's already on the ball. I don't have to hit through the ball quite as much. I'm redirecting it. I'm going low to high really fast to get that topspin to bring the ball up and down quickly. Often I hit this shot because I'm late to the ball, so my contact point is behind where it would be on a forehand drive. However I also hit a buggy whip forehand on putaway shots sometimes, where I can make contact a little earlier, but I still using that drastic low to high brushing action, which results in the so-called "reverse" finish.
I'm no expert, but to me the shot seems a little wristier than a normal forehand, I feel like I'm a little late to the ball, and I'm using a little more wrist action to control the ball and compensate.
Players like Nadal and Federer that have really good running forehands still try to contact the ball somewhat in front, and hit through it somewhat while also getting all that brushing action. A player like Sharapova, doesn't have quite as a good a running forehand because she often lets the ball get too far behind her. Then again her movement is an issue so that could be part of her problem.
If you try to contact the outside of that ball more you'll get that sidespin that hooks the ball back into the court.
I think it's a pretty simple shot. No need to make it too complicated.
Edit: Another great situation for this shot is when you're hitting a passing shot. If their approach has any pace on it, you can use the buggy whip forehand to bring the ball down quickly and put it at their feet, and also to hit a more extreme angle.