two-finger backhand grips yield a much greater amount of tendon flexibility. they are also much weaker. the strength aspect is a double edged sword: people who throw two-finger must have better timing to throw successfully and so most people who throw two-finger are better able to manipulate the disc's weight shift naturally (than people who throw with 4) and get the "edge around."
the downside: two-finger grips have much lower strength potential.
with that in mind, while a higher percentage of two finger grippers may throw 350'+, it is more difficult to throw 450'+. i will often train people to throw with two fingers (to develop shifting the disc's weight) and then have them add four and tell them to "do the same thing you did with two." in most cases this yields an increase of 30-50'.
basically: a two-finger grip makes it easier to feel when the disc's edge should come around. a two-finger grip makes it harder to transfer maximum power when you bring the disc's edge around.
After reading this thread, the preference of a four finger power grip makes a lot of sense. The key to strong snap is acceleration. With four fingers underneath the disc, you are getting a micro-burst of acceleration as the disc rips off of each finger, beginning with the pinkie. By removing a finger from the grip, you are losing a degree of acceleration.
sort of... but for different reasons.
if you think about the hammer drills and the "reverse thumb lead" methods, the premise is to widen the radius of the spiral of the disc's outer edge as it comes around from the start of the power pocket. e.g. there's a sepcific point on the disc that you want to rotate ~180+ degrees... basically it starts on the back side of the disc and comes around to the front side of the disc before leaving. to make it easier, assume this motion is a perfect "half circle" arc. you have the starting point, the finishing point, and the apex of the arc. the acceleration builds the most as the point passes through the apex of the arc and completes the final 90 degrees of rotation (this happens when the hand shifts from ~12 o'clock to ~4 o'clock). as the point passes the apex it will be pulling outwards (intertia) with a tremendous amount of G-forces. this is when 99% of slips/micro slips happen. the stronger grip makes it easier to hang onto the disc through that final part of the arc.
i won't describe this in more detail unless you actually hold a disc (put a sticker on it at the point behind the hammer's head) and trace through the arc to see what i'm talking about.
with the hammer/thumb lead throw, the arc isn't a perfect half circle, it's oblong, more like the hypotenuse and short leg of a rounded edge right triangle (but slightly less extreme). this basically makes it a "sharper turn" that the point on the disc has to make as it completes the arc and increases the G-forces/inertia even more. if you let the disc "swing loose" in the pivot, there's almost no way your rip point can hang onto it well beyond the apex.
i will diagram these differences... if/when someone actually holds a disc in their hand and traces out the motion and describes that they are seeing the point on the disc moving through the arc.
to make a comparison to something other than disc golf:
the pacing of the path of the point on the disc is a lot like making a high speed turn in a car. if you enter the turn too quickly (without control of the car's weight) you will skid and hit the wall (aka the understeer will pull you out). your goal is to enter the turn slow, exit the turn fast.
a strong grip is like having the batmobile and firing a cable/dart into the ground on the inside of the turn so that will hold the car around the turn at high speeds. if you can do this, you will actually accelerate coming out of the apex of the turn. there is a limit to grip strength that is equivalent to "breaking the cable" and sliding out and hitting the wall if the disc/car's weight is out of control.
