cutting scar tissue to get more scar tissue

I hear of cutting scar tissue off the leading edge to stimulate more scar tissue to grow to cover a large wound, yet have never seen an up close photo of how much (or even how often) to cut. It seems if you cut to much or to often that you will actually lose more than you gain. Does anyone have any up close photos explaining this process.
thanks,
alex

hi,
just scratch the edges with a sharp knife to continue callousing. if you hollow the wound properly with concave/knob cutters the callous usually keeps rolling towards the middle without further damaging - takes quite a long time on a potted bonsai though

thank you very much, so just scratch it to expose green? Just like when you scrape with a thumbnail to see if something is still alive? How often do you do it and is there a certain time of the year to do it in? Im trying to get a 4" diameter convex cut on a bald cypress top to cover. I have it in a large tub and have let a sacrifice apex grow rampant for 2 yrs and it seems that even though the exposed wood is plenty hard the callous rolling has not even changed an 1/8"