You have two sails in concentric rings that are connected, with your craft attached to the outer ring. When you want to decelerate you disconnect the two rings and and tweak the reflection of the inner ring to be somewhat outward focused and give the inner ring a little push. As it moves forward the energy bouncing off the inner ring is directed backwards to the outer ring, which slows. As long as the inner ring reflect more energy backwards (and to the outer ring) than the outer ring is getting from your source the outer ring slows down. Really hard to describe, but easy to draw. First proposed by Robert L. Forward.
