Shift-Spacebar is used in the Reader, and it is Word Dictionary focused. The most common use of shift-spacebar is to mark a word in the Reader as Known. It accomplishes the same thing as turning on the Known checkbox in the Word Dictionary window. With shift-spacebar, however, the word dictionary entry is immediately hidden.
Shift-spacebar can also be used in the Reader to show a hidden word dictionary entry. When a character is selected in the Reader that is also part of a known word (underline not shown), pressing shift-spacebar causes the word dictionary entry to be displayed. Pressing the spacebar is similar, except that it also shows the character in the Character Dictionary window.
Taking this a step further, if a Known word entry is shown in the Word Dictionary window, pressing shift-spacebar causes the word to be displayed and selected in the Known list of the Reader (Known button tab would be depressed), with the entry hidden in the Word Dictionary window.
Pressing shift-spacebar again redisplays the word in the Word Dictionary window.
Finally, pressing shift-spacebar once again removes the word from the Known list. The word is marked as Unknown with the Known checkbox in the Word Dictionary window unchecked.
Much more information and examples of using shift-spacebar are provided in Tutorial: Reading with ChineseToolbox.