Not sure if you mean this, but remember, after reading somewhere, to finish adding or expanding a selection, must press F8 or SHIFT + F8 again..I think it was something like that.
Not sure though, don't have an imac to try it out. Try at your own risk :).
I would be happy to know if I could help with this information.
Nikolino
I know I don't know anything (Socrates)