My bad, it's not the tab button but rather the backspace button to switch to the scuba diver view.
I had to look in the source code to figure out what it was.
I'm assuming that it's a bad sign that I couldn't remember how I coded my own easter egg.
Also as a sidenote, since I'm giving away easter eggs... In the Black Cauldron, if you have the sword you can kill the geese and the goat back at the cottage for a couple of extra points (the goat made me mad one to many times). Also if you swing the sword 3 times on the screen with the castle as the background you get a thundercats print statement and a quick view of an elongated sword held up in the air. I never got the thundercats symbol to look right in the sky but I had planned on it being there.