Hi there! So I did some research (found the original article years ago and can't find it now, buried in some forum no doubt) and it appears that with the razor scrape the grower is exposing the phloem just under the outer bark.
Here is where it starts to get divided among opinions as to what happens next. Some say that there is more surface area for the rooting hormone to get in (which I find hard to believe since there is just as much surface area than before the scrape) and the other school of thought is that when the phloem is exposed it simply lets the roots grow out faster since this outer layer has been removed. I subscribe to the latter thought.
Either way just scrape the stem where it will be under the surface of whatever you are using to root into (where the stem will be "stuck into" under the rockwool or peat pot). This is a very light scrape and with a sharp flat razor one will see thin bits of outer stem come up in practice.
I used to have a cloning kit that was nothing more than a men's travel shaving kit (with the rooting hormone powder residue it looked like a cocaine kit haha). This was really handy since the mirror in it helped by providing an area to do the scraping and the cutting that was easily made sterile with rubbing alcohol. Since I only grow for myself I have evolved (some would say devolved) to autoflowering plants so my cloning days are over for the time being
It is fun to do though, but really tries one's patience in the beginning!
On proofreading I see this message is full of parenthesis, sorry LOL Don't know what happened here...