Mix groups in Pro Tools are very useful but it can be annoying when you have the group enabled and you go to tweak the level of one of the tracks in the group only to have all the faders move as one - as they should. That's what a group is supposed to do, you just forgot it was enabled.
Users of Avid Control surfaces will know that, rather than suspending the group, you can just move the fader while touching another fader at the same time and the group will be temporarily suspended. This is called "clutching".
To do the same thing with only a keyboard and mouse just hold Control (Start on a PC) and you will be able to move the fader without all the others moving, but only while you are holding Control.