Interference when two keys are rollover simultaneously

dnssec-key-rollovers note There are never two key rollovers running in parallel for one zone. If a rollover is triggered while another is in progress, it waits until the first one is finished.

I found that if there is a zsk in retire stats, ksk can still rollover, but if there is a ksk in retires-active, the rollover of zsk is delayed until the ksk stats is updated。 So what states do zsk and ksk have that affect each other's？