The index finger acts like the nut or zero fret when you barre. Picture the guitar neck getting shorter each time you barre the index finger. You theoretically "move" the zero fret (index finger) up towards the body of the guitar.

Also, a lot of players tend to use the thumb by wrapping it around the neck. Although I deem this unorthodox, many players do it. Jimi Hendrix would use his thumb, as a lot of blues guitarists. In some circles, it is deemed not correct. However, I've studied classical guitar and yes, that technique is a no-no, in modern and some contemporary music, there are players who incorperate the thumb.