Each submitted share will increase the credit of the miner who submitted the share by the share difficulty.
The miner who accumulated the most credits will receive the reward of the next mined block and his credits will be reset to his current credits minus the credits of the runner up miner.
Re-setting the credits of the miner who did receive the block reward to 0 was abandoned as it did penalize miners having an above average hashrate.
Usually a miner will receive a full block reward as soon as his accumulated credits equal the current block difficulty (+/- pool luck).
Uncles are distributed randomly amongst all active miners on the pool. The credits of the Miner receiving the uncle reward will not be reset.