The kids assembled a double-acting piston and it has a flow meter on it. I noticed it has a little doo-hickey down inside it that appears to be threaded, so I presume by twisting said doo-hickey, the flow rate will change. But what da heck are you supposed to turn that doo-hickey with? A special tool that we lost somewhere? A pair of tweezers? 
We just use a small screwdriver
The one we have does not have a slot, instead it has a metal tab that protrudes, but it’s down inside the durn tube thing. 
Hokay, so I just tried to move the tab around with a screwdriver… anyway… and, yes, it works… kinda… I just thought maybe there was a tool we were missing.
when we do it we just use a pair of needle nose pliers I don’t know if that would work better or not than the screwdriver though
I’ve done it with an Allen key before, by basically sticking the key to the edge of the tab and maneuvering it around. It took me forever, but it does work if you don’t have a pair of pliers handy.
You dont need a sunken tab to use a flat head screwdriver.
Place the flathead into the hole, and just turn counterclockwise. The more you do this the faster the rate of airflow to the piston.
By golly, I learn something new every day! 
Thanks, everyone.
Also if you take off the flow meter you can unscrew it correctly through the hole that goes into the piston.
I usually just use Allen wrench like stated before but I remember someone telling me that you can unscrew from the other side.