Hooking (or hook breathing)......
When you surface begin to exhale before you hit the top such that as soon as your airway clears the surface you can begin to inhale, i.e. about 3 feet from surface......
Once you take your first breath in, hold it briefly, close your mouth and compress the air, this whole process should take about a second - dont hold it for any length of time.....
Apparently it speeds the transfer of the oxygen into your system, as this transfer takes around 8 seconds on average.....
But the aim of the game is controlled performances - with a buddy, so you should in theory never be that close anyway...