It means playing with your two index fingers only.Fluffyumpkins wrote:What is Index?
With this style you normally play on the four arrow keys.
This is as opposed to spread, where you generally use both index and both middle fingers on 4 different keys, usually something like Z and X and 2 and 3 on the numpad, depending on what's comfortable for the user.
Index is generally regarded as more difficult than spread because you only have the use of two fingers (making it so each finger has to be responsibly for two arrows) and because of the layout of the arrow keys, which makes crossovers actually difficult, whereas in spread they're the same as any other stream.