The more you close it (less air), the drier the skimmate will be (less water).
Opening the air valve will make the skimmate have more water in it (wet skimming).
Roger usually recommends to set the Tunze skimmers to skim "wet", meaning more water will be in the skimmate. Aim for something like a light colored Tea.
I don't know of any scientific studies, but people usually say skimming wet pulls out more crud and more phosphates (even though it looks watered down). Even if skimming wet doesn't pull out much more "stuff", it will help keep the skimmer neck clean which is very important for operation.
There a no dumb questions!
Jeff