from: http://eligius.st/wiki/index.php/Shared_Maximum_PPS

"Miners accumulate Pay-Per-Share as usual. When a block is found, the pool counts the total unpaid PPS credits. If there are sufficient pool funds earned to pay them all in full PPS, that happens. If not, the miners are paid proportional to available funds. Remaining pool funds accumulate toward future payouts. The difference between a miner's actual (SMPPS) earnings and PPS earnings is retained as "extra credit", and considered in future blocks."