What happens when you press the gas pedal and brake?

Another use of pressing the brake and accelerator together is called a “Line Lock”. It is usually used in rear-wheel drive cars to warm up the tires for better traction.

Do you mean pressing the brake and gas together all the way down?

That is right but keep in mind that those drivers push brake and gas simultaneously to trim down the extra time shifting foot. You should not try this with your traditional automobile as it may damage the whole transmission system. This is also done by car racers.