How does Android detect USB connectivity?

I'm going to put a tablet in my car to accommodate my many desired functions. Of these functions is a backup camera. I plan on using a Logitech webcam that I had around for the camera. The webcam uses USB, so I was going to get an OTG Y adapter so that I could charge the tablet and get data from the camera to the tablet at the same time. This would require a Kernel Update, but that's already lined up. I plan on wiring the power for the webcam into the reverse light wire (either using a relay or a DC-DC converter), and then wiring the data cables to the front of the car to the tablet. This way, the camera would turn on only when the car was in reverse, and then I would have Tasker launch a video viewing app once it detected that the USB had been plugged in, which would be initiated by the webcam receiving power and sending data. This is based off of the assumption that Android recognizes a USB device being "plugged in" as the transfer of data being initiated, **or** power transfer being initiated. What I want to know is, which one is it? **Is USB detected by whether or not power can flow from the host to the client, or whether or not the tablet is receiving data?** Because the tablet would only be doing the latter, and power of the webcam would be completely separate of the tablet. I suppose I could test this, but I wanted to check here first before I murdered several USB cables in the process.