Snap rolls are not reversals. At the end of a snap roll you're generally headed in the same direction as when you start. Reversals are divided up like this. First, role reversal (notice I said role, not roll). This is simply going from offensive to defensive or visa versa. Second, control reversal. Changing from a left turn to a right turn. However, the OP asked specifically about reversing or turning back quickly.
This, simply defined, means a 180 degree change of direction. Holms is correct as far as he goes. Your best turn rate occurs at corner velocity so if you're above corner you need to slow down (high yoyo), if you're below corner you need to speed up (low yo-yo) and if you're at corner then, theoretically you can just do a flat turn. Problem is, with WWII aircraft you'll always bleed so a flat turn isn't going to do much good. There is another consideration though and that's your turn radius. A high yo-yo, even if below corner, may be advantageous because it uses gravity to decrease your turn radius. If you have a large turn radius and are being chased by someone with a smaller turn radius give you by the high yo-yo will reduce the angles he'll gain and may even result in a neutral nose-to-nose pass. If you do a low yo-yo, gravity will increase your radius and may give your adversary the ability to turn inside of you.
There's another subset of reversal's though and that's the vertical reversal. These are usually also known as a "rudder reversal." In this case, we're talking about a vertical fight versus a predominently horizontal fight. The typical scenario is one aircraft roping another. When the lower fighter falls off the upper fighter reverses and dives in for the kill. This reversal is typically at slow speed (below about 100mph) and is best done using a combination of rudder and engine torque (hence the term rudder reversal). At higher speeds (above about 100mph) then simply completing the top of a loop works.
