Here’s a picture of a rower, just to explain the basic concepts. Click on the image to see a full size version. The system consists of a crew with its mass and a boat with its mass .
The entire system is moving with a velocity with respect to the viewer on the dock. However, the crew is moving with respect to the boat, and as a consequence (conservation of momentum) the boat itself moves at a higher speed . The crew velocity is .
Conservation of momentum gives us:
So, we can derive what you do to your shell speed by racing the recovery: