Bionics allow a Space Marine who has suffered a crippling injury to return to service, but are unlikely to improve or enhance his abilities. However there is a chance an attack or shot will hit a bionic part causing less damage, eg a shot that would cripple a leg will only cause mild damage to a bionic leg. To represent this, if a model with bionics is killed, instead of removing it, place the model on its side. Roll a D6 at the start of the next turn: on a roll of a 6 the model is stood back up with 1 wound, but on any other roll it is removed as a casualty.
Bionics
Source: Codex: Space Marines, p. 34