You shouldn't be worrying over trivial things like the break time and the exact procedure and stuff. Just stay calm and be yourself. Take deep breathes and release all the air from the lungs in a quick whiff. Repeat it four to five times and you'll feel relaxed naturally.
So here comes the actual part. Only after you click "Yes" does the break timer start. And then, you have to raise your hand and a person will come to your desk and escort you out of the test-zone. Then you'd have to do your thumb and palm prints again and then, your break starts. When you report back to the test-zone, everything above starts with frisking you, if need be, to check if you do not carry anything illegal into the test-zone and then the palm and thumb prints will resume. Meantime, just make sure, you take a good leak, drink some energy fluids, have some protein bars or something and most importantly, DO NOT PONDER over the last section, all within a stipulated 5 mins, leave a gracious 3 mins to the security routine.
Actually my experience showed that the time starts immediately when the screen pops up asking whether you want to take a break or not. I find this ridiculous, and complained to GMAC about this. Of course, GMAC jumped at the opportunity, and solved the problem immediately...NOT. They didn't do anything. My issue was that if the timer starts immediately when the pop-up screen comes, by even bother with the question to begin with of whether you want a break or not. Secondly, there is no time or timer on this screen, so how are you supposed to know how much time is left?! Ridiculous. By the time you are escorted out of the room and scanned out, you lose an additional 15-30 seconds, and don't expect your proctor to tell you how much break time you have left when you are taking you break, because they claim not to have that information on their screen. I find this pretty ridiculous, and instead of the GMAT testing my logic and thinking, it is testing my time taking abilities. Also, you aren't allowed to carry a watch or any other time-keeping device, and looking at your cell phone during the break is a big no-no as well.