>"How do I replicate this with another (non-debian) distro ?"
Any distro set up similarly should do it. If anything, it's the xfree86/window manager combo that handles this.
>"What causes this behaviour ?"
The keyboard itself. When you hit alt+number on the separate number keys, you alter the signal that's sent to the keyboard controller on your motherboard. Because old PC's were intended as cheaper versions of IT mainframes, they included a number of functions that programmers happened to like. (Nobody, not even IBM, truely expected "lowly" secretaries would ever use a PC, despite the way it's portrayed these days.) With this feature, if you had a problem (in ancient, ancient programs), you could sometimes alter the way the program ran by entering a code this way. Later, it became a way to extend the limitations of the keyboard, and then it drifted out of use with the introduction of multitasking systems.
This is because there wasn't much need of it with a multitasking system. If your program does something unexpected, you simply open another window to alter it's function, or kill it. With older "real mode" programs, there was no way to signal the processor to alter it's function (aside from just killing it, which sometimes didn't work if the processor locked up), as the processor is only doing one thing, running your program. This was a way to bypass that, provided the programmer built the functionality into the program. This never really made it into the mainstream, even later on, because it added a lot of code to programs that were barely small enough to run in the first place. Well, that and it opened a big door to anyone that wanted to screw up your data.
Actually, I'd like to know if this is the same for USB keyboards as well, as they don't necessarily have to support "legacy" applications.
>"How do I get this to work with KDE ?"
Don't, know, I use GNOME.
It should be relatively easy, however.