The following description of the switching block or belt-block (the term "belt-block" has reference to the diver wearing the valve block attached to a belt near the front of the diver's body) has been taken and modified from Dave Sutton's IDA-72 Teardown pages:
Here is the unique belt-block. As you can see, it is fed with a gas-line (the green-banded hose above) and two lines running back to the rig. The two obvious selector-valves here are lifted about 1/4 of an inch before they may be moved, and then move through about 120 degrees of arc. One simply purges the loop, offering a high-flow bypass of the internal mass-flow orifice. The other drives a pneumatic piston in the plenum chamber of the rig. When this valve is activated a piston moves about 1/2 inch and breaks the loop into two halves, thus making the rig an open-circuit system. The diver inhales from the auto-add valve and exhausts into the water through the normal exhaust valve. This instant conversion mode of the rig from a rebreather to an open-circuit system is VERY clever.