Second optical sensor in 430EXII - What's it for?

By being a complete pillock I've managed to sever a cable in my 430EX II.
It's a black optical fibre that runs from behind the lenses in the flash head to the circuit board on the back of the flash (the one that handles the LCD screen and button presses).

It's not for the wireless system as that goes via the sensor on the front of the body (according to the manual anyway), which leaves exposure.
All the explanations of E TTL flash that I've read say metering is handled by the camera (which is also suggested by the TTL part of the name) so what does the sensor on the back of the control board that this fibre runs to do?

I can only guess it's to do with knowing how much light has been emitted by the flash, but wouldn't the flash know that already given that it controlled the output in the first place? Or is this sensor used to judge how much light has been outputted so the flash knows when to stop?
Perhaps it's just for the exposure indicator light in the pilot button (though I think that's too much to ask)?

The obvious thing to do would be to try the flash and see what's broken but I haven't got immediate access to a soldering iron to solder the other wire at the moment.

Does anyone know what this second optical sensor is for?

I think there is a link to the service manual somewhere on here just not sure where,might be worth a bit of searching on the lighting section.

I did have a look for service manuals but I couldn't find one. With you saying that I've looked again with a slightly wider search but I still can't find any.

There was a thread with a link to a 430EX parts catalogue (which was dead) and a 550EX parts catalogue. The 550EX link still worked but it didn't cover wires or what circuit boards do, it's just parts and their numbers.

This is from the 580EXII service manual regarding replacing the fiber wire:

NOTE 9 ：Replacing the Fiber Wire
① Cut the existing Fiber Wire close to the connector.
② Connect the end of the new Fiber Wire to the remaining wire with tape. (Fig. 033)
③ Slowly pull on the existing Fiber Wire to pull the new Fiber Wire through the Pivot
Ass'y. (Fig. 034)
④ Remove the tape and the existing Fiber Wire.

My first guess was that it might be for the flash's built-in evaluative metering, but the 580EXII's manual clearly shows that sensor as being located on the front below the AF assist beam emitter. So I don't have an answer for you on that, but the above instructions seem to imply that it's acceptable to splice two pieces together, so if it was me I think I'd tape those two pieces back together and call it fixed.

Thanks for posting that.
The 430EX II doesn't have the built in metering like the 580EX II does so it can't be that.

I read that as saying use tape while pulling a replacement wire through the assembly then throw the tape and the old wire away, rather than taping a new piece onto the old piece.

Where do you get service manuals anyway? I've never seen anywhere credible looking selling them.