The same modifiers will behave differently, depending on when you use them. Notice how in the above examples, you're instructed to only press the modifiers after you've begun to drag? There's a reason for that. Pressing the modifiers before clicking causes completely different behavior.
If you already have an existing selection on the screen, you need to decide if you want to replace it with the new selection, add a chunk to it, or remove a chunk from it. This can all be done with modifiers, as well. (Note: this also works with the shapes tools).
- To replace an existing selection, do nothing. Create it as instructed above.
- To add to an existing selection, hold down the SHIFT key before clicking and dragging. Once you've began dragging, you can then continue to hold SHIFT if you want the addition to remain symmetrical. If you want it to revert to free form, release SHIFT once you've started dragging. You can also add ALT into the mix if you want to expand from center.
- To remove portions from an existing selection without destroying the whole thing, hold down the ALT key before clicking and dragging. Then, just like above, either press or release SHIFT and ALT in combination to achieve the desired results.