Diplomat not emptying fully?

I've had this problem before and the guy I called out showed by a knob under teh front of the machine, said to turn it clockwise with pliers. I am trying to remember 1) why and 2) whether I shluld turn it back again.

I think the reason was a build-up of deposit in the drain pipework and turning the red knob somehow forced a proper drain - and started a cycle too.

I have got it doing a drain now and will try returning it to it's original position when it is finoshed. but would appreacite a bit of advice about what I've done and why it's fixed it!

Thanks in advance