A stake pool always has an "active stake" and a "live stake". "Active stake" refers to the amount of ADA that is used for rewards calculation in the current epoch. "Live stake" is all ADA that is delegated to the pool, also counting ADA that was delegated/undelegated after the "active stake" snapshot for the current epoch.
At which point is the snapshot for the "active stake" taken? Is it right when the new epoch starts? And will this happen at the exact same moment for all stake pools?