Every vehicle has a Boost Gauge in the bottom centre 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 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 little longer or shorter to charge an overcharged turbo because the engine is revving up slower.
When you charge up your boost with the Super Diesel or with the Racing Truck and if your needle is stopping to charge the Overcharged Turbo, you can even spam click the left pedal in very small intervals and your turbo will still keep charging.
Note: That 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.