[Faraday]

Код:

if(IsPlayerInAnyVehicle(playerid) && GetPlayerState(playerid) == PLAYER_STATE_DRIVER)
	{
		if (PRESSED(KEY_CROUCH))
		{
    	new Float:x, Float:y, Float:z;
			GetVehicleVelocity(GetPlayerVehicleID(playerid), x, y, z);
			SetVehicleVelocity(GetPlayerVehicleID(playerid) ,x ,y ,z+0.3);
		}
	}

That's how I do it.
Add it in OnPlayerKeyStateChange