Yayox said:and how it benefits you in handle?
it maks the car sideways right?
Complitely the opposite! Imagine you're going in 4th and brake HARD before the corner. You need 3rd or even 2nd to make good exit, if you press the clutch, engage gear and release the clutch while cornering, you have big chance of ending sideways because of momentary change of torgue on rear (I assume RWD) wheels.
The only right way is to select a gear before the corner, enter the corner braking and smoothly accelerate out from the apex. And since shifting 4->2 isn't trivial while braking hard from high speed, you need to rev the engine just before you release the clutch to match it with gearbox and wheels.
So while you're stepping on brake with your right foot's toe, you press the clutch, move the shifter, momentarily blip the throttle with a heel and release the clutch while engine is revving.
Sounds simple (well, maybe not) but actually it requires lot of practise When I try that and fail to blip the throttle, I usually panic, go into the corner with clutch depressed, because I'm afraid to release it and end up sideways, and release it only at the exit. Loosing valuable time
More often I just brake earlier so I have some time to change a gear, because it's slower without heel&toe.