Marines rally as they fall back.
A single squad can be forced to make multiple pin checks in a turn, or a pin check and a morale check. You may not make a last chance regroup attempt on the same turn you were defeated in an assault and forced to fall back. Morale, Pinning and Regrouping are all Leadership tests, Psychic tests are different, and do not count as a Ld test as far as the rules are concerned. Multiple wound creatures count as a number of models equal to their current number of wounds. Voluntary fall back may be used, but only if the unit in question is engaged in HTH combat with a model it has no way to hurt - for example, Imperial Guardsmen in combat with a Wraithlord would be permitted to use voluntary fallback.