Does this mean:
a) even if in combat; or
b) even if enemy wholly or partially intervene between them and the arriving flankers?
a) seems to be imlied, and I don't remember anywhere where it might be explicitly contradicted.
b) seems to implied, and, at least in the case where there is complete intervention, seems a little harsh to me. It would seem that, for example, heavy Impact Foot lining up to charge some nice juicy mdium foot in open ground could be forced to flee by the arrival of a few skirmishers to the rear of the MF. Sounds bit far fetched perhaps, but now I've said it, it's bound to happen to me in Leeds
OTOH, Maybe it's historically the case that even this would be enough to spook the HF?