Really simple question here.
How does pricking or fudging tighten the stitches? Pressing down on the welt would seem to make the stitches looser, if one managed to move the leather much at all.
If one wanted the stitches tighter, why not pull them tighter when stitching rather than go back later and fudge or prick?
I kind of answered that in response to another post but fundamentally the stitches, by tradition are supposed to be proud of the welt surface. Master DA Saguto says that all the old, dead guys--the elder and eldritch shoe gods-- admonish against pulling the stitches too tight. They're not supposed to be loose, mind you, but they will sink into the welt and not be as visible...and put more strain on the welt...if they are pulled too tight.
Neither pricking nor fudging will do much to accentuate the stitches, short of damaging the welt, if the stitching is pulled tight
When you prick up a welt, a large part of what's happening is that leather is being displaced upward underneath the stitch. This tightens the stitches, if nothing else does.
I would be speculating by offering any other mechanism but this is what has been Traditionally ascribed (passed down from master to student, if you will) to pricking--separating and tightening the stitches.