You're sitting in a car. The car has helium balloons tied to its floor. The windows in the car are closed and locked and the Air conditioner isn't working.
What will happen to the balloons when you press on the gas?
Will they move forward, stay in place, move backwards...?
The answer is they will move forwards.
Because helium is lighter than air, it will go where air isn't. When the car is pushed forward the air moves backwards, so the helium balloons will move to where the air is not - forward.