Skimming Modes & the Red Dot

I've been familiarizing myself with iMovie '08, and there's some behavior I don't quite understand. Why is it that, when you're editing a title or adjusting audio, a red dot appears in the middle of the play head (or whatever that vertical red bar is)? And when that red dot appears, skimming only works when holding down the mouse button - i.e. when dragging the mouse as opposed to simply moving it over the clips?

I can't figure out what the significance of this "mode change" is and have come up empty-handed when consulting the documentation. Does anyone have any clue as to the significance of this change in functionality and play head appearance when adjusting audio or editing titles?

-Steve