Open the black screw (or thumb screw on some tools) on the back (seen in the top image above). This controls the black stop at the end of the jig. Lift the bar at the top and insert your tubing (as seen in the bottom image). Place the bar in the down position to hold the tubing in place with your thumb. The smallest slot on the rectangular block (bottom image) is where you saw the tubing. Adjust the screw in the back for where you want to saw. The stop allows you to cut as many pieces at that length as you want. I like to wedge the red handle into the "V" section of my bench pin for cutting.

Tube rivets are especially lovely connectors. Making Metal Jewelry Book includes a section describing how to make tube rivets.