Toolbox tools -Merge Mode functions

Several of the toolbox tools -Smear, smudge, brightness, blend etc have Merge Mode settings.  These are Add, Subtract, if lighter, Logical functions etc.  None of them are grayed out so I assume that they have functions. 

I can't get any of them to do anything "logical". If I get anything, it is usually a dark or light stripe.
I can't find any mention of these in the online help.  Any help from users?
Thanks --Pat