I think what they are talking about with spillover is, for instance, when at the end of a passage you play and hold a note with reverb and delay. While you're holding the note, you use your foot to prepare for the next passage which uses a different group of effects and settings. You want to go ahead and press the switch to make the changes, but you also want the reverb and delay from the previous setting to continue while you're still holding the note.
That's my guess. I don't use my setup anywhere near what it's capable of doing, although I would like to. But every time I sit down to practice, I'm usually focused on just learning how to put my fingers on the right strings and in the right frets cleanly.
In glancing at the Fractal stuff, one thing that is enticing for me is the ability to set it up with your computer using USB. In theory, that should make the programming a lot easier and quicker.
