Fascinating. I always wondered if this sort of thing was possible when I noticed as a child that pressing multiple buttons at the same time on my parents calculator made the screen show stuff that wasn't actually real numbers (things like a 2 with a part missing, etc.). Likewise seeing which buttons on the VCR have precedence (e.g. if I hold down "play" and press "stop" then what happens? And vice versa?). I always assumed the device wouldn't be designed comprehensively to handle all possible inputs like that, so there was a chance some of them would allow you to do funky unintended stuff.
Given the complexity and freedom of access that a videogame has, I'm not surprised that this hack is technically possible, but it is very impressive that someone's managed to do it!
There's this kind of unintended effects all over the place in tech.
A particularly interesting one for programmers might be the M6502 CPU.
All opcodes on the M6502 are 8 bits. Due to the way it was implemented in order to keep transistor count low, various patterns will trigger specific functionality at specific stages of execution (you can find a javascript emulation of it that displays the execution in excruciating detail as the result of creating a transistor exact clone of the design by decapping an actual 6502 CPU and scanning it...).
These were arranged so that the documented instructions present a suitably useful instruction set. But all of the remaining opcodes still does something that was simply deemed pointless by the designers.
Some have been found useful by demo writers in particular as a way of saving cycles. Some are just totally bizarre and/or unstable. Some does fun stuff like putting more than one value on the memory bus at the same time. Some even locks the CPU up so solid it needs to be power cycled to recover...
Here's an overview of the nitty gritty details from someone who actually knows what they're talking about: http://www.pagetable.com/?p=39
Ha! When I was a kid we had Pong. You know, the hard-wired kind that plugged into your television, and get off my lawn, but this one had three variant games: Pong, basketball, and ... racketball I think. There was a hard three-position switch to switch between games - and if you positioned it just right between positions, you could coax the game into a hybrid game that mixed up the court layouts.
Given the complexity and freedom of access that a videogame has, I'm not surprised that this hack is technically possible, but it is very impressive that someone's managed to do it!