This is crucial, users want this option.
Mouse Hover Activate Listers in Dual Mode (whatever lister the mouse is over is automatically activated [becomes source lister]).
So users using an optical mouse (using the middle scroll button on their mouse) could scroll one lister (source) and then just move his mouse over the destination lister and IT now becomes the source. The user can then continue scrolling (interact) with the new source without first having to click anything.
Say I have a button on my toolbar that targets selected files, if I want to process files in the destination lister, I first have to activate this lister (by hotkey, clicking in the lister, or clicking the taskbar. NOW with Hover activate, I just have to pass my mouse pointer over the destination lister without clicking anything, it becomes the source lister. This is a cool way to also avoid inadvertently messing up your selection as you click in the destination lister(I'm aware there are other options around not losing your selection because of a mistaken click).
It's ACTIVATING the interface. Fields could also respond to the mouse hover to become active for typing, etc. It makes the interface snappier. Users had a need for this since OPUS 6.5