tion again as a pulse when you release it, or you can set
   it up to <i>only</i> perform its action when released.

   <li>Toggle: the button toggles its virtual state between ON and OFF each
   time it's pushed, so that it acts more like a slide switch than like a
   pushbutton.  This can be useful for buttons that control modes: push the
   button to engage the mode, push it again to disengage.

   <li>Shift: the button acts as a modifier, changing the meanings of other
   buttons while being held down. 