The Power Unit bar refers to how much power your car has.

The Power Unit bar in the game starts at 70% and recharges at varying rates depending on the engine mode selected. In the neutral engine mode, the bar recharges at a moderate pace, providing a steady power recharge rate. The power save engine mode recharges the bar even faster, but using the fast engine mode depletes the Power Unit rapidly. As long as the Power Unit bar is not empty, switching to the fast engine mode boosts the car's speed and increases the chances of overtaking opponents.

When your car's Power Unit reaches 0%, it means that you have run out of power, and your engine mode will automatically switch to neutral. The fast engine mode cannot be used until the Power Unit recharges to at least 5%.