The older videos of Borger show him regularly using a thumb-on-top grip, but the three point grip he now advocates (forefinger on top) is the one described by his son, Jason, in this Flyfisherman
article: The Three-Point Grip | A problem-solving grip that can make you a better caster.| 1
The pictures with the article illustrate exactly how the grip is accomplished.
The three main grips out there seem to be the traditional thumb on top, the "v" grip with the thumb and forefinger on either side of the handle, and the three-point grip with the index finger on top. The three-point grip does assist in not allowing the wrist to 'break' during the backcast, thus leading to a better casting stroke - IMHO.
Most people I know use the thumb-on-top grip and are very successful with it. My son does not like the three-point grip and sticks with the thumb-on-top, so I guess its really a matter of personal preference. I feel I'm a better caster since learning the 3-point grip and I use it almost exclusively. When throwing large streamers with my 8-wt and double-hauling line to a distant target, which takes more arm and body movement than when using my 5-wt for trout, I usually switch to the thumb-on-top method to get the strength, control of the powerful stroke, and the power needed to make the longer casts.