I'm working on a carb + knock box + ICM setup right now. I took my knock box apart to sort out this T10 thing.
My box is part number 811 907 397 E. The one with yellow lettering. Cannot guarantee the internals are the same for the other one(s).
T10 is connected directly to the collector of an NPN transistor, a BC337 to be exact. The emitter is connected directly to ground (T3).
This make sense, as MacGruber stated, it drives the low side of the fuel pump relay. I can only assume this is done so the fuel pump does not run all the time when the ignition is on and the engine is not turning. The knock box does this rather than the fuel computer as it's the one with the RPM input.
Time for some assumptions and a hypothesis.
Assuming I am correct in that the fuel pump will be shut off in the absence of RPM. I also don't think these cars prime the fuel system, otherwise there would be no point to the fuel accumulator, the knock box could issue a prime pulse, I'm not sure. This means when you turn the ignition on you will see 12V on T10 through the fuel pump relay coil while the pump is off. While cranking you will still see 12V while the knock box gets synced to the RPM signal, and then probably a bit of a delay in your meter. Once RPM is detected and the transistor is turned on current will flow though the relay and you will then see 0V on T10 just about at the same time you stop cranking. This makes it look like T10 is receiving power during cranking and I bet the origin of the powered T10 myth.
What does this mean for people who connect T10 to 12V? You killed the transistor and it doesn't do anything anymore. Which is fine because if you weren't using T10 as originally wired you don't need it anyways.
Unless of course there are different knock boxes that do different things with T10. In which case this only applies to the E variant. Everyone send me your knock boxes so I can take them apart!