Well it could be something like the collapse / expand button which hides all Tasks except the one selected.
A cool addition would be to make the selected Task bigger as well.
Sorry for not being able to provide a visual example at that point.
I found a good explanation in the Omnifocus Forum:
when you want to isolate a single task, you drag it to @!focusedview in the sidebar
navigate to @!focusedview and do the work
after work is done, mark task complete and move the next one
In any case, I’m gonna try this myself to see how it works. I tend to have a bunch of stuff in my “Focus” list, so another level of focus might be useful.
The ! in the name so that it’s always at the top of contexts.