I found out about this text selection technique by accident, but found it to be very useful.
Let’s say you are writing a document in TextEdit —or any other text editor, for that matter—, and all of the sudden want to change the style of the headings or paragraphs in your document. If your text editor does not have styling build into it, that formats all of your headings, paragraphs and quotations, etc., you probably know how tedious it is to go through each of these elements, changing their style independently.
To do this more efficiently, you can use a keyboard command, which selects text in a non-linear fashion. Try selecting a piece of text, like say a heading. Headings are usually not ordered in your text side-by-side, meaning that there is probably text in between them. Hold down CMD + OPTION + SHIFT, now you can select another heading, regardless of whether or not there is any text between the two headings you have selected. You can now bring up the text formatting tools and format all of your selected headings accordingly.
This becomes particularly useful when dealing with text editing applications, that lacks a document styling features — like TextEdit.
Another technique, which inverts your selection, can also be quite useful at times. Holding down CMD + SHIFT will select text, just as regular SHIFT-selection. However, if you do this over already selected text, the previously selected text will now be omitted from your selection.
There are a few other related keyboard commands for text editing available. Some of which, I do not know the application for, but try them out for yourselves:
Box Selection: Hold down the OPTION key.
Box Invert Selection: Hold down CMD + OPTION.