While a shift is open, any blocks the pool finds are paid out to all open shifts (10% of the block per shift). This means that you will continue receiving payments on completed (open) shifts even if you stop mining.
With PPLNS, each open shift is paid the following:
((Block Value + Transaction Fees) / 10) - Pool Fee (3%)
This amount is then split evenly among all ~40 million shares submitted during the shift.
PPLNS involves variance: If the pool is lucky, you can earn a lot more than normal. If it's unlucky, you will earn a lot less. In the long run (a few months or more), you can expect to earn approximately 5% more using PPLNS compared to PPS.
They have a minimum payout of 0.01BTC on top of that.