it needs a solenoid and a switch onto the throttle mechanism, and a vac pipe linked up (i forget what else).

you have to use a twin piston dump valve or it will sound like a knat farting.

if your engine is otherwise standard and sounds tractor-ish, and if your car looks blatently standard and diesel-esque, it will be daft. they sound good on a tuned up turbo-diesel with a beefy/burbley exhaust. dont be tempted to use a cheapo small dump valve.

the derv doctor will do you one but its about £200-300 supply and fit.

also you can rig up a switch on the dash, to the power to the throttle switch, then turn it off when you dont want it.