Once you have sufficient Melee/Unarmed skill, you unlock additional, special attacks with those combat skills, both in and out of VATS (to use them out of VATS, you need to do a power attack - hold the attack button down, in combination with moving in a direction for varied results).
Thing is, the actual effect of these special attacks is... horribly undocumented, and the Wiki isn't quite up to speed on them either.
I've figured out that the 'Back Slash' with sword/knife weapons just costs less AP, but are there any special effects? Is there any good reason not to just spam them in VATS? (Out of VATS, they tend to be substantially slower than normal attacks, for better or worse I've found).