I've finished Insanity before, it's probably the best workout if you're looking to lose fat, however you will definitely lose muscle and you can potentially be burnt out.
I think I know what you're question is, basically you're asking during the like 3-5min interval of working out if you can rest, well basically what I did and what you must do is that if you do rest, you rest after maximum efforts/failure
E.g. its like 3minutes of activity vs 30 second rest (when you're supposed to), if you rest in that 3 min period of working out, you make sure that you put your best efforts in and that you worked till failure, then after a brief period of time you get back into it and repeat that until you're actually supposed to rest.
It's hard to explain, but basically what I can tell you right now is that it comes down to mentality, it's that mental willpower that's going to keep you going, if you rest for 1 minute in that 3 min burst of activity you're kidding yourself, just go until failure, I was pretty athletic when I did it so when I worked till failure, I only had like 5-10 sec rest before I pushed it out, and the black guy's voice also motivates you, so s'all good