How does air transport through the 2 holes other than the one that one that is hooked up to the on off switch?
I’m guessing you’re talking about the solenoid valve, since the “T-fitting” is pretty self explanatory.
There are two different solenoid valves, one for single-acting cylinders, and the other for double-acting cylinders.
The valve for the single acting cylinder has an input for supply air, and a connection to extend the cylinder. When the solenoid is off, the supply air port is blocked, and the cylinder connection is vented to exhaust. When the solenoid is on, the supply air port routes to the cylinder connection to extend the cylinder.
On a valve for the double acting cylinder, the supply air port is always routed to either one or the other cylinder connections, while the other cylinder connection is vented to exhaust.