Going the same direction as the swell is a blast (as long as it's a swell, not a cresting, breaking wave). If you're racing downwind towards a race mark (gate marks), try taking your mainsheet block off of the ratcheting mode and into freewheel mode. As the swell gets close to you, there should be an increase in wind speed. When you start to feel the sterns lift up try sheeting in the main just a little to pick up some boat speed. With practice you'll get really good at this. And, by having the ratchet turned off nobody around you (if racing) will hear you sheeting in. If on the other hand you have a large breaking wave coming up behind you, forget what I just said and get your butt on top of the transoms steer straight & hang on.
