We use PPLNS reward system. Our pool calculates the percentage of shares you send in the last N shares. Block reward is shared between miners proportionally to this percentage.
Depending on the pool hashrate it takes some time (usually a couple of minutes) for the total amount of N shares to come up.
Therefore if your rig switches off a couple of seconds before the block was found - you would get you reward completely (as it was turned on). If it turns off 15 minutes before the block - you get nothing.