Greg,

When this circuit is open (no continuity) rpms will rise by about 300.
In a manual tranny Mustang, the tranny switch and clutch switch are in parallel.

Real world scenario is this: Trans in neutral, switch continuity, engine at low idle. Press clutch to the floor,clutch switch makes continuity, shift into 1st gear, no continuity on tranny switch, still at low idle because clutch switch is now making continuity. Begin to let clutch up, switch opens, this tells ECU to anticipate load to prevent stalling (Feed Forward Signal) rpms come up. I believe the ECU begins to advance timing resulting in rpm increase(I'll try to prove this next time I have a timing light hooked up).
In a stock Mustang, I think the clutch switch has two sets of contacts. The other works as a start interlock(safety) by interrupting the circuit to the starter solenoid.
If you don't already have it, you should get this manual.
"How to Understand,Service and Modify Ford Fuel Injection & Electronic Engine Control", by Charles O. Probst, SAE

Steve
P.S. One other thing I finally got to work is the "Check Engine" light in the Volvo cluster. In the past the only way I knew I had a code was to periodically check with a code reader.