Every vehicle has a Boost Gauge in the bottom center which indicates the current boost pressure. The dial will raise from zero to full as long as you hold down the accelerator uninterrupted. Releasing the accelerator or braking will lose all boost, even in the air.
Once the boost gauge is full, the needle will eventually begin to shake if you have the Tuning Part "Overcharged Turbo" equipped. This means that the boost pressure is close to release. Shortly after this, if you do not interrupt your acceleration, the boost pressure will release and your Overcharged Turbo will engage, giving you a boost of speed.
After releasing the boost pressure via your Overcharged Turbo, you will need to release the accelerator or brake in order to begin charging your turbo boost again. This works unlimited times, but it takes every vehicle a different amount of time to charge an overcharged turbo because the engine is revving up at a different rate.
When you charge up the boost with the Super Diesel or the Racing Truck and your needle is stops charging the Overcharged Turbo, then you can quickly click the brake pedal in very small intervals and your turbo will resume charging.
Note: without the tuning part "Overcharged Turbo” equipped, there is no way to release the boost pressure and you will not receive any speed boost for hitting max boost pressure.