Homebrew PCBs

I've been looking at Volkan's tutorial and I'd like some clarification
on a few points:

1- why does the paper needed to be emulated... won't the PCB tray act
like paper?

2- which sensors will need to be emulated?

3- Are there any more specifics on what needs to be monitored?

4- how is the moved optical sensor used.

Any other clarification would be helpful as well.

I have the repair manual for this printer, but I'd like a little bit
more on the theory behind the electronics modifications.  If I can get
a bit more information, I'd be interested in releasing a step-by-step
how-to on the printer, complete w/ my own source code and
schematics/pcb layout to help others create functioning PCB printers.