The black plactic tubing that comes with your Isspro boost gauge gets inserted there. Just push the tubing in till it bottoms out and pull back slightly. All done and connected up to 250 psi. To remove the tubing, pull down on the green ring and pull the tubing out. You have to pull the green ring down pretty hard to get it to release. No need to pull the green ring down to INSTALL the tubing....only to release the tubing. Alot easier than messing with the little brass wedding bands that come in the kit. If you don't tighten enough or too much they leak or they compress the black tubing some. These new connectors are slick as heck and make connecting a boost gauge very easy. Edited by: Maverick