I think Greg’s definition of “cool in, hot out” is more accurate than “slow in, fast out”. You really want to be “fast in, fast out”.
The definition of “racecraft”, as opposed to just being able to drive, is knowing how and when to attack the entry or exit of a corner depending on the situation.
Minimizing the amount of time you’re on the brakes (in other words, braking as hard and as quickly as possible) obviously will be the fastest way to slow down. So in hard braking situations you’ll want to be on the brakes hard and fast as much as you can. But in racing situations or times where you aren’t slowing down that much, maybe you’re going to be more strategic with your braking force. Sometimes all it takes is a little graze of the brakes in a long corner to set the kart nicely. Or if you’re trying to set a guy up into the corner, maybe you’ll really slow down your entry and go nice and easy to fully maximize your exit.