In Core, Overtime is basically expected in every game unless a team is insanely bad at the game. If both cores are still alive by the end of the timer, even with a sliver of health, Overtime is triggered, and Core healths are reduced by dividing max health with current health. Normally, max health is 25, but in Overtime it's 6. Round it down to 24 and divide it from there: above 20? You get 6 health in Overtime. Below 4? You get 1 health. That's it.
EDIT: I may have gotten some numbers wrong -- it's been a while since I played BlockWars.