You can check off completed ToDo items by pressing the spacebar. The HP 200LX User's Manual and the Help Screen in the Palmtop tell you to press the Minus (-) key to check off completed ToDo's. Neither source says anything about the spacebar.
However, the use of the spacebar toggle is documented in the HP 95LX manual (page 17-30). HP must have left the spacebar toggle in place so people who upgraded from the 95LX wouldn't have to unlearn one keystroke and learn another.
-Victor Roberts 70413,1423, Jeff Mattox et al