*Numbers portrayed may not be 100% accurate*
The DSP switch is nothing but an adjustable "load on a circuit". Typically a switch/fuse/fusible link etc are not loads, but these ones have built in resistors in them. Depending on how you have your tunes set up, I'm going to say tune 1 is all the way to the right, in the switch that may have the least resistance from source voltage to ground showing .03v-.9v drop. Position 2 has a little more resistance, 3 has more resistance, 4 has even more and 5 has the most. You see a numeric voltage on the computer because the ECM is seeing a voltage drop across the circuit from this "Load" like you mentioned above.
For Example:
Tune 5:
Receives a 5v DC signal from the ECM, has the most resistance in that part of the circuit, the computer shows you the difference which is approx 4.0v-4.9v. Whenever your are doing a voltage drop the DMM or in this case ECM shows you the difference between the 2 points, not how much voltage is there.
Tune 4:
Receives a 5v DC signal from ECM, has a little less resistance than position 5 causing a smaller voltage drop. Should see 3.0v-3.9v
Tune 3:
Receives a 5v DC Signal from ECM, has less resistance than position 5 & 4, should show a reading of 2.0v-2.9v
Tune 2:
Receives a 5v DC Signal from ECM, has less resistance then positions 5,4,3. Should show a reading of 1.0-1.9V.
Tune 1:
Has the least resistance in the circuit. Will show a voltage drop <1.0v
Voltage is electromotive force. Voltage pushes amperage (current) through a circuit. Every time voltage encounters some resistance it gets used up. In a circuit, you will see source voltage all the way to the break in the circuit, or open in the circuit if its not grounded. So by adding more resistance with the 1-5 on the switch the ECM is knowingly sending out 5v, and is monitoring how much its receiving back, and makes its changes based on that much like a Throttle Position Sensor. If you see source voltage on the ground side of a circuit, you have a bad ground connection, or a dirty ground because now the ground is acting like a "Load" on the circuit and its using some of the voltage the switch was supposed to use.
Correct me If I'm wrong, but that is at least how I believe they work. I could be totally out in left field wearing football pads to a softball game. Let me know if I'm wrong or anything.
