Never seen the effect, but I think he's describing the phenomenon of surface flare. Some developers tend to work mainly at the very surface of the film emulsion, where there's an optical effect that tends to spread any bright light hitting the film.(posted 8069 days ago)
I've heard quite a few reports of the effect, but never experienced it myself.I seem to remember that some Glycin and Pyrocatechin developers are supposed to exhibit the effect as well, but my memory might be playing tricks here.