Howdy
Excellent question!
With a heavy recoiling revolver, if the middle finger is right behind the trigger guard, or in contact with the trigger guard, then every time the gun recoils the trigger guard will whack your middle finger right on your knuckle. It hurts and after a few times you will develop a sizable flinch.
That is part of the reason modern revolver grips are shaped the way they are, to get the shooter's middle finger away from the trigger guard.
Like this:
Even the old Tyler T-Grips helped keep the trigger guard from whacking the knuckle.
With the typical Semi-Auto, the way the grip is shaped, it is impossible to get your finger behind the trigger guard, so it will never whack you.
But revolvers are different, most of the traditional designs will whack your knuckle very well if the gun has a relatively strong recoil.
This old 38 does not recoil very badly, so it does not whack me when I shoot it.
These 45s, on the other hand, will whack me real good if I grip them with my middle finger right behind the trigger guards.
Which is why I always tell new revolver shooters, forget what you learned about shooting the 1911. Forget about gripping it as high as possible to minimize muzzle flip. That is a great way to get a black and blue knuckle. With any Colt SAA style gun I NEVER try to cram my entire hand onto the grip. I ALWAYS allow my pinky to curl under the grip. That allows me to shift my grip down a bit, leaving about 1/4" or so of space between my knuckle and the trigger guard. I can shoot those Colts all day long with full house Black Powder loads, and it does not hurt a bit.
Being a traditionalist, I do not like modern grips on older revolvers. I prefer to keep the historically correct grips on them. With this Model 28, the gun is heavy enough to absorb most of the recoil and the trigger guard does not whack me.
But with a K frame 38 like this:
Or even a relatively mildly recoiling 44 Special like this I prefer to keep the correct grips on it and hang my pinky under the grip to keep from getting whacked.
Anyhoo, that's the deal with modern revolver grips that keep your fingers away from the trigger guard.