The tool is just an Ideal Tire Groover. It came with a #4 head and blades (#4 translates to 1/4"). I set the depth at 1/4" as well and just cut where you see in the pics. There are lots of YouTube videos and writeups on the web, and my technique wasn't really anything special.
Nice job - I'm going to do the same to mine, from what I have read it makes a difference on the R4's - I'm fortunate as I do not have any steep grades to worry about so a little will be just what is needed... I have to look at the groovers so happy to here the Ideal worked for you....
I have a groover, looks like an old soldering iron. Add a blade, plug it in, wait for it to heat up, cut. If you go too fast the blade will cool off and become very difficult. Works fine.
Friend of mine has a groover that is commercial grade. Cuts however fast you want to go. Only heats when the blade is depressed and activates the electrical short designed to create heat. Let off and it stops heating. That thing will wear you out.