If you finish a section early, you DON'T have to continue on. If you continue on right away, that extra time is lost. But if you just leave it at the last question, you can chill or go walk around or whatever for your remaining time.
Like for me, math was ridiculously easy. We get 45 min, I was done in <20. So I took a nap in the hallway. The 45min writing went by in like 30 min I think, and I just wandered around in the halls for the remainder of my time.
It really bothered me that the GRE doesn't have any builtin breaks, so I created my own... lol. Just make sure you get back to submit your last result!