perhaps better labeled "exhaust retarder".
I use it on my Canter manual to maintain appropriate speed downhill, select the gear at which speed will be maintained. Very effective. All downhill. Particularly on slippery slope where touching brakes can create problems.
Useful in traffic, but needs a gap to vehicle in front and looking/planning/predicting well ahead. Require brakes for last bit of stopping. Heavier the traffic the more reliant on normal brakes I am.