[emilamlom]

They can be used to decrease orbit as well. Since you just need to bleed off the speed from Earth's orbit, you could angle the sail diagonally so the the reflected light is pushing against your direction of orbit (sort of like how the fins on a pinwheel are angled).

While I was googling, a couple places likened it to tacking into the wind, but that's a different kind of phenomenon that works because of friction and pressure differences.