Blowoff valves
 

What do they do and how do they work ? :O

When you let off the throttle, The turbo is still pumping, So it lets the air out. If it doesnt get out, It can backwash through the turbo and cause it to slow down, and/or break. Plus they make a cool sound. And they work kind of like an engine valve, If youve ever seen one. Except instead of a cam they are opened by a solenoid or vaccum actuator