how do T-fittings work?

how do they work without spilling water all over? do you fill the system so that it doesn't go that high? or do you cap the end of the fill line?

You cap the line. It's usually never filled all the way to the top anyway.

yeah, you cap the line, although you can fill it with the thing running if you're just using an aquarium pump. anything that isnt over like 1000gph wont have a chance at pushing water out of the end of the thing while the system is running. I cap mine for security and transportability

depending on where/how you have your T-line installed, it can shoot water out if it is open. If you are unsure, just cap it with your thumb when you first turn it on. Slowly remove your thumb, and you'll see if it's gonna squirt out or not. When the system is filled and bled, it is best to cap your T-line for security.