Sorry may be using wrong terminology, but what mean by slicking is that I get my leather wet, usually by very quick dip, the apply a lather of dove soap and then take a glass slicker an with moderate force run it all over the front surface of the leather. This smooths out imperfections and compresses the leather prior to tooling. I don’t do this on all pieces, but really like what it does to softer belly and shoulder leather. You may get some stretch when you do this so best to do before final cuts. Really like how it tools after compared to loose spongy leather. I know one saddle maker that compresses all his leather with a press prior to tooling.
This isn’t burnishing and not on the rough out.
Kelly