So I consider myself a reasonably competent mechanic and will happily get stuck in with most jobs. So there I was the other day dropping the rear wheel out of the Kona so I could clean up a sticky piston in the brake calliper. I ended up pulling the wheel in and out a few times and each time cursing like mad and wishing I had three hands as I battled to get the axle past the rear mech, the cage and the disc in to the calliper all at the same time. Each time I kept seeing the small button on the SRAM Rival mech with a little padlock drawn on it. I fiddled with it once or twice, but couldn't make it do anything meaningful, so I just concentrated on the task at hand.
After packing up I decided to consult google to sate my curiosity and discovered that it's actually a nifty way of locking the mech cage in the forward position to reduce chain tension and negate all the problems I'd been having! Cunning really, but I wish I'd known about it before I embarked on my work. Ah well always next time.