If a unit moves into cover that lies within its fall back corridor, it may attempt to regroup again as it does so. Effectively this is a second chance to regroup. A unit may only attempt to regroup in cover once per movement phase as long as none of the normal restrictions apply. Make the test once the whole unit is in cover. If passed it regroups and halts. If it fails it must complete its fall back.
Regrouping In Cover
Source: Core Rulebook 3E, p. 73
A unit that falls back into cover within its corridor gets a second, once-per-phase chance to regroup as it enters the cover.