Double clutching is not necessary with modern-day transmissions. They teach that at the driving schools because the open wheel cars they use have synchro-less trannies. All you do is blip the throttle between gears to try and match revs. You only depress the clutch once. Blipping ensures a smooth downshift and prevents possibly having the rear end come around on you during sudden unmatched engine braking. You're supposed to downshift and brake hard in a straight line before your turn-in point and then while turning towards the apex immediately get power back to the wheels and accelerate as you pass the apex to maximize exit speed (more important than entry speed). Some people teach trail-braking (light braking in a turn) to help balance the car depending on the car setup and the turn. Sorry I got kind of carried away. Just my 2 pennies.