kbde-driver is linux kernel module for emulation 
keyboard input.

Linux work with REAL keyboard in the following way:

Linux 2.2 and 2.4
   from the keyboard_interrupt function called 
   handle_kbd_event, which call handle_scancode
   with scancode parameter.

After scancode has been got from the keyboard hardware
all real work done by the handle_scancode.

So, kbde-driver do very simple thing:
for each character which was written to the /dev/kbde device
it call handle_scancode function.

Linux 2.6
   there is a serio_interrupt function (really placeholder)
   which invoked with scancode parameter from real interrupt function.

   kbde-driver for each character written to the /dev/kbde device
   call serio_interrupt().

That's all, but it's work.