The theory, backed up by no end of animations but zero actual film footage AFAIK, is that the first blade pulls the hair out a bit then the second blade snips it a bit lower than it would otherwise have been able to reach.

Blades 3 through N have never been explained to my knowledge.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8