The basic notion for this pen-based text entry
system is that a character is "typed"
by first moving in one of six directions, and
then moving again in one of six directions.
That allows a virtual "keyboard" of 36 characters.
The way you get things like capitals
and numbers is by entering special characters
that "escape" to other virtual keyboard layouts.
Typing the most frequently typed characters a,e,i,o,t,SPACE requires only straight line movement, which is faster.
The thing I like about this approach is that it opens up the door to all kinds of rapid-text input devices. Six is a really small number, and it takes very little time to choose one of only six directions with a joystick, or hit one of six buttons on a portable hand-held device. On a six-button input device you would "type" by hitting two buttons in succession. But things get really fast if you have a two-handed device.
For example, one could imagine a PDA in which you type with your left and right hands at once. Under each hand is a small key pad with only six keys. You enter a character by simultaneously touching one key on the left keypad and one key on the right keypad. To type the most frequent characters you hit the same key with both hands, which is really fast.
The two-fisted approach also works If you're on a game console, where you have a joystick or directional thumb toggle under each hand.