[1] It is similar in concept to the fission-fragment rocket in that the fission by-products are directly harnessed as working mass, and differs primarily in the way that the fragments are used for thrust.
In the fission sail, the "rocket" is built in the form of a two-layer sheet, with some sort of absorber on one side, and nuclear fuel on the other.
In the simple case where the decay releases the fragments "front" and "rear", the rearward moving fragment generates thrust directly, while the frontward moving one is absorbed in the front half of the sail with no net contribution to thrust.
Close to stars where the light density is high the sails work fine, but as they move past about 2 AU their thrust is too low to be useful.
Forward suggested that coating the sail with fissionable material would provide thrust in this region, where maneuvering is no longer as important.