I found
an article on flytyer.com showing several "hot spot" flies. Some of the flies remind me of a local Wisconsin pattern, the
pink squirrel nymph, which is my all-time favorite subsurface attractor fly.
I haven't done a side-by-side comparison, but I can guess at why it works well. We have two jobs as fly anglers: 1) get the fish to see the fly, and 2) get the fish to take the fly when it sees it. The hotspot surely aids in job 1. For job 2, keep in mind that the question a fish is usually asking itself when it sees a fly is not "is this food or an artificial lure?" but "is this food or debris?" We try to use flies with features, like overall shape, movement, and translucency, that are often found in real insects but rarely in little pieces of twigs or grass or other debris in the drift.
One of those features that's fairly rare in debris, I think, is a sharp transition between two contrasting colors. We see it a lot on real insects, for both camouflage reasons and coincidental reasons (like black wingpads on a ready-to-emerge nymph, or the contrast between the head/body and case of a caddis larva).
I think regardless of which colors/shades are involved, a sharp contrast between light and dark is a lifelike feature that helps the fish separate prey from debris. And I think when fish aren't being selective to a single hatch, they're mainly looking for any combination of lifelike features as a positive trigger to indicate that something is food, or at least likely enough to be food that it's worth grabbing and then spitting out if it's not (they do this all the time with real debris, and aren't thinking about hooks).
So the "hotspot" almost certainly helps the fish notice the fly, and is probably more likely to help than hurt when a non-selective fish is deciding whether to take the fly. I'd say it's worth keeping a few in your box.