The Fusion drive is an incremental improvement over the older Nuclear engines.
Working on a similar principal to the venerable Fission drives, the Fusion power generator instead of breaking apart large atoms joins smaller ones together.
This fundamental difference allows not only for a greater energy collection rate due to the subsequent 'free' electrons (which themselves can be used to continue the reaction), but these fused atoms can be re-used to a certain extent further increasing the available output.