The NOS/Recvr menu and Driver HUD option toggles the Forward+Backward Arrow key controls to either Nitrous Boost or Recover.

Nitrous Boost is an instant blast of power, like a temporary turbo boost. The strength can be adjusted via the > Controls, > Turbo menu or directly in the Root Settings script. You can also assign a sound to this in the Root Settings or by drop-in sound assignment

Recover lifts your vehicle up and un-sticks you out of hazards. There is also a button for Recover on the Driver HUD that always works even if your NOS/Recvr is set to nitrous boost. You can adjust the key-activated recover height in the Engine script approximately line 37

float           RecoverHeight   =   1.5;