Hmm, actually if Jeff shades the project before uses the antique, it sounds like he uses a method more similar to what Indy described -- using dye to darken the areas you want to look shaded. . .
However, if it comes off so easily with a wet rag that certainly indicates to me that the antique does not appear to be soaking into the leather; and if it is not soaking into the leather (i.e. actually coloring the leather), you may end up with the stain cracking and/or coming off the leather when it is bent later because it never actually colored the leather; it just sits on top of the resist. . . It is important that the leather itself is actually colored -- not just coated with an antique of some sort to make it look right. . .
It is easy enough to make the leather look similar to this (Jim Linnell artwork), but I have yet to find a way to color the leather in this manner as opposed to just putting an antique on over a resist and hoping the stain it does not crack and come off later. . .
Anyway, that's how I understand it. . .