You get paid for the average hash rate over X amount of time. X being 5nd. So if the time you mined for falls within the 5nd and a block is found then you get your share of the block. As soon as the period of time that mined falls outside the 5nd, you no longer get paid for your shares. So if you stop mining, you will still get paid for that last share for 5nd after you stop.

The easiest way to "see" it is go to the "Shift Graph" on Kano.is and look at the shaded red area. That is the 5nd for the last block found. Any area in front (to the right) of that graph is new work that will be counted once a new block is found. Any area behind (to the left) of that graph is previous work outside the 5nd.