Sorry, it's been a while since I read about it and I'm having trouble finding the info again. I imagine it's similar to the stroke-based Chinese entry systems because each character would then have an unambiguous key sequence, obviating the need to keep checking the screen and selecting words from the menu.

e: Actually I found some info here: https://ja.wikipedia.org/wiki/%E6%BC%A2%E5%AD%97%E7%9B%B4%E6...

It looks like some systems are T-Code, TUT-code, etc.