You have to go, it's time to go, and if you don't go then you can't go until the next window.

The moment in time when you need to poop and you know for a fact if you do it now it will be a good poop. Waiting past this window results in a less then perfect poop, Usually with not getting it all out or straining harder than you would have had to if you had done it during your O.P.W.