As mentioned on the Help Payouts page here, we use PPLNS (Pay Per Last NShares) The N value the pool uses is 3 days. When we find a block, you are rewarded for your part of all the work done on the pool by all miners, in the 3 days up to when the block was found. Your percentage of all the work done by everyone on the pool for the 3 days determines your percentage of the reward.
When you are logged in, look at your Workers ShiftGraph here with the following explanation: The "User Shift Reward Performance" Graph shows a graph of your recent shifts on the pool. On the graph it shows a green area on the right, of the 3 days of shifts up to now, with a red line across it, showing your average hash rate in the green area. Thus if we found a block now, your rewarded hash rate would be close to that average hash rate shown. Each point on the graph is your hash rate for each shift.
How does PPLNS work?
The easiest way to understand how PPLNS works is to consider the green area your mining tank. Each time a shift completes on the pool, the new shift is added to your tank, on the right, pushing all the previous shifts to the left, and the last shift on the left in the green area, gets pushed out of the tank. The new average hash rate will be the previous hash rate, plus your new shift added on the right, minus the old shift pushed off the left. This is exactly what happens each time a shift completes on the pool.
So from the above explanation, if the hash rate pushed off the left is lower than the hash rate added on the right, your average will go up. Conversely, if the hash rate you add on the right is lower than the hash rate pushed off the left, your average will go down.
When you start mining on the pool, your tank is empty, so each shift added on the right makes the average go up. After 3 days of mining, your red average line should be close in value to your miner's hash rate, as long as it's been mining the whole time. Each shift after that, the average should stay around the same value, as each new shift added on the right will be close in value to the shift pushed out off the left.
If your hash rate goes down, it will take 3 days for the red average line to drop down. If you add more miners, it will take 3 days for the red average line to move up to the new rate. Any change you make to your hash rate, will take 3 days to complete the change.
So, for example, if you drop your hash rate by 100TH/s, for only half of the 3 days, the red average line will only drop down half of that, or drop by about 50TH/s, by that time. However, as mentioned above, it will still take 3 days to go back up, if you add all the 100TH/s back on the pool.