Actually, it doesn't . . . but it depends on which object is highlighted. Space generally activates a behavior (the default behavior) of an object. In this case, I suppose that most general items do not have a Space/Shift-Space function and it drops down the scroll bar. However, for example, highlight the word STORYHOURS in megamania's .sig, then press TAB to highlight the Show button (spoiler block). Now when you hit Space, it shows the spoiler rather than paging down. Shift-Space does the same thing. Strangely, links seem to require the RETURN key.
So, basically don't get into the habit of using Space because it won't always work and may in fact activate a button in the web page you are on.