WikipediaのSpace-cadet keyboardの項によると

> Each group was in a row, thus allowing easy chording, or pressing of several modifier keys; for example, Control+Meta+Hyper+Super could be pressed with the fingers of one hand, while the other hand pressed another key.
ttps://en.wikipedia.org/wiki/Space-cadet_keyboard

と書かれていて、modifier keysは文字キーを押す反対側の手で鍵盤楽器で和音を弾くように押すものだったらしい