Here's my take on that one. The clip allows the knife maker to drop the point so that it is in line with the centerline of the knife for a more positive thrust and allows the sharpened point to be convexed, or at least less obtuse than a sharpened swedge on a drop point, like a dagger tip. It makes a more devastating cut. Kinda like hawkbill designs but not as exaggerated.
I believe Bill Bagwell described it as increasing the pounds per square inch slashing force delivered by the tip in the back cut.
True about centering the point, although many clips don't bother doing that, which is odd...
The whole point of a concave curved clip is to allow a broad blade to have less overall point taper, and so more full width blade mass forward, while a deep concave point-centering clip will still allow for a fine profile point within the short remaining length: The knife then has both forward blade mass for utility chopping, and yet retains a certain stabbing ability.
A fine point knife will always feel blade light while chopping, while a truly dedicated chopping knife will always tend to be blunt pointed (the Busse Battlemistress for instance): These things are in opposition. The deep concave point-centering clip is an attempt to bridge both aspects (successful in my view, although I find straight clips sleeker-looking, while I recognize them as less functional).
A concave clip is inherently less sharp than a convex spear point clip, so it makes more sense on big chopping knives.
As for Big Bagwell's "backstroke", I see it essentially as flicking the wrist to stab sideways from an unexpected angle: It can be done with most sharp point objects, and the concave clip was not absolutely necessary to do this, unless the blade was very broad.
Gaston