(But what's the little dash all about, and why doesn't Keyboard Viewer show it?)
If you watch what's happening in the ruler, you'll get a huge clue.
What's happening is that you're invoking outline mode. Option-tab gets you into outline mode; within outline mode, option tab (or plain tab at the beginning of a line) indents the current line, moving it one level more subordinate in the outline. Shift-tab (or return at the beginning of a line) does the reverse, moving the line one level less subordinate. Move out enough times, and you get out of outline mode.
The formatting is controlled by the "Lists" popup menu (or by the Format->List... menu item), which lets you specify independently for each level how list items at that level are formatted. Items at each level can be labelled by consecutive letters (A, B, C, ...) of various types (uppercase, lowercase, greek, arabic) or numbers (1, 2, 3, ...) of various types (decimal, roman, japanese, arabic). Labels can be surrounded (or replaced by) arbitrary punctuation (leading/trailing parentheses, periods, quotes, angle brackets, whatever). The default is to label each line of the outline with a single hyphen.
The reason you can't see the hyphen used for formatting in Keyboard Viewer is that it's just formatting; it's not coming from the keyboard.
