It tightens the stitch in the same way that you tighten a stitch when you pull from the bottom. Except instead of that pulling action, the prick pushes the thread. Personally, I do think the stitch prick tightens the stitch...esp. if, as DAS says, the top (welt-side) stitch is not pulled as tight as the bottom stitch. The prick tends to regularize the stitching on the welt to form "beads" or as I remarked fairly early on--a "string of pearls." But more to the point I have always made...even if you see no tightening, why do you need to "automate" such an inconsequential technique. What purpose is it addressing? What question does a fudge wheel answer that is not, at least and maybe better answered by a welt prick? Speed? Convenience? If welt pricking has no original purpose then certainly fudging is nothing but spurious ornamentation. And in my opinion, FWIW, I think fudging looks plastic and mechanical...industrial even...maybe esp., when it is damn near perfect.