Strat: Middle Only
Analysis:
The middle-only position is a bit complicated, so bear with me. To start with, both wires from the middle pickup go to the phase reversal switch. This is a DPDT switch mounted on a push-pull pot. In my application I have this one on the volume control, but it doesn't really matter. This switch reverses the signal and ground wires, which puts the middle pickup in and out of phase with the other two. When the middle pickup is being used alone, as in the current scenario, the position of this switch has no effect on the sound.The one output of the phase switch leads to the series/parallel switch, and he other leads to the zero-terminal of Pole-2 of the five-way (upper right). Let's start by following the signal path through the 5-way switch: First of all, the middle-only is position 3, and we can see that Pole-2/Terminal-3 isn't connected to anything, so nothing is happening there. Pole-2/Terminal-0, however, is also wired directly to Pole-4/Terminal-3, which is connected directly to the output (Pole-4/Terminal-0) in this position. Thus, we see that one of the wires from the middle pickup is going directly to the output, and nothing else is connected to the output.
We're not quite done yet, though. Remember that the other wire from the Phase Switch is connected to the Series/Parallel Switch. In order for things to work properly, we must ensure that this wire is connected to ground, regardless of which position the Series/Parallel Switch happens to be in. Notice that the terminal at the upper right of the Series/Parallel Switch has two wires connected to it. Thus, the signal coming from the Phase Switch will be present on the second wire regardless of the position of the Series/Parallel Switch. Let's trace the signal through this second connection.
After leaving the Series/Parallel Switch, this wire leads to terminals 2 and 4 of Pole 4, as well as terminal 3 of Pole 3. Since we know that the switch is in position 3, terminals 2 and 4 are not connected to anything. Terminal 3 of Pole 3, however, is connected to terminal 0 of Pole 3, which, we see, is connected to ground. Thus, we see that for any orientation of the Phase and the Series/Parallel switches, one wire of the middle pickup always goes to the output, and the other always goes to ground, which is what we want.