[ezmaass]

Using launch control, if your car is equipped with it, is hands-down the best option if you're looking to maximize your acceleration from a full stop. It essentially will hold the engine RPM at a specific level while standing still (higher than idle) and allow you to "launch" off the line well within the power-band. You can somewhat simulate this even without launch control - keep your left foot on the brake and using your right foot to rev the engine slight to around 3.5k - 4k RPM, let's say, releasing the brake and mashing the accelerator at the same time. But if you have launch control, use it - the computer will do a better job.

Kick-down is not for launching, it's simply for highway passing. When you push the accelerator down, you'll feel there's a point of slight resistance where you can "click" passed at the very end of the pedal travel. It's the equivalent of mashing the pedal to the floor. That point of resistance, once exceeded, is known as "kick-down" and tells the transmission to give you the lowest possible gear at current engine speed.

As for manual versus automatic mode, I'd just leave it in automatic. You'll need Sport+ for launch control (follow the instructions in your manual). Shifting in manual mode has no benefit as far as achieving the best acceleration time goes... you're not going to do any better than the computer, and gear changes will happen at the same speed no matter whether you're initiating them or the computer.