Originally Posted by DannoXYZ

It depends upon which way you rock the bike. What Compressed mentioned is when you rock the bike away from the power-stroke leg. So if your right-leg is on it's downstroke, you rock the bike to the left. This requires extremely strong arms and back to counteract the leg-muscles. This effectively raises your gearing as it artificially shortens your crankarm-length. I think this is the only scenario where rocking the bike helps.