Exhaust throttle?

[LouM]

Hello im new
I've just joined, my name is Lou and I'm hoping someone could help me understand what the strange plunger/box looking thing is attached to the exhaust on my 02 vito petrol camper.
I've been told it's an exhaust breaker, throttle. I have no idea what it does.

 

[whitenemesis]

Increases engine braking, for long downhills? Saves the brakes overheating

 

[Petrol Pete]

Exhaust brake . Not quite the 'Jake Brakes's' fitted to big rigs, firing compressed air into the engine to slow the truck down. Not sure I have ever heard one in the UK. Bloody noisy on older trucks