The basic method is this: Whenever a block is found, payment is given to shares in a window, starting with the last share submitted and going backwards up to some number N of shares. Shares older than the window are not paid. This is essentially a cutoff decay function. There is no concept of rounds – shares can be paid even if they were found before the previous block.
This means that a given share can be paid more than once – however, the payouts are chosen so that the expected reward is equal to solo expected reward. Because the payment for a share depends only on blocks found in the future and not on shares and blocks found in the past, there is no way to hop based on the current status of the pool.
However, if implemented incorrectly, it is possible to hop based on imminent difficulty changes. The most naive implementation is to have a fixed N and simply pay the last N shares equally. However, this makes it more profitable to mine before difficulty decreases, and less profitable before difficulty increases – the number of blocks that are expected to be found in your window depends on the future difficulty, while the payout you can receive elsewhere depends on the current difficulty.
It doesn’t help if N is chosen to be a given multiple of the difficulty at the time a block is found. If the difficulty is about to increase, it is more profitable to mine a short while before.
Should I choose PPS or PPLNS?
For example, if N is set equal to the difficulty D, a share submitted D shares before the increase will be paid the full expectation in the D-window, and then when difficulty increased it will once again go inside the window and have more expected reward. And, like the previous case, shares submitted just before the difficulty change will be rewarded similarly to shares submitted after it.
In addition to the problems above, it has a problem common to all reward systems that use time as a factor, which is that it is more profitable to mine when the current hashrate is higher than the average. The correct method is as follows.