I'm just getting back into cycling and there are a lot of things that I've forgotten and things I just never knew about. I bought a mountain bike the other day and the left handlebar has this RevoShift Friction thing on it. It's not numbered. It simply has minus and plus. I haven't tinkered with it because I don't know what it'll do. If someone could let me know what it does I'd really appreciate it.

Its the front shifter - friction means that it doesn't have the clicks for each gear.