I take it that it's a 1.9D or a 2.4D
It's not a "choke"
It slightly advances the injection pump timing to aid cold starts, in reality you shouldn't need it in this country but...
Them b*stard 2.4's are a bit kak TBH and they don't like starting when they have a few miles on their back, and if the pump timing has been done by some idiot then chances are you will need to use the lever most of the time.
All the lever does is move the advance piston at the bottom of the injection pump, once the engine has gotten over about 1500 rpm the lever does f*ck all... but..
I have driven vans, and not just a couple, where the lever being pulled out has made a marked difference in how the van performs (is that the right work used in the same context as a 2.4D?) I can only presume that the injections pumps are worn and don't produce enough internal pressure to move the advance piston as far as it should, hence driving with the lever pulled out could actually advance the pump timing over a greater rev range.
never looked that closely into it to be fair, but it's not a choke.