Several folks have mentioned that the "cut" the groove for the wire ribbon, and indeed they did cut it however I don't want anyone to get the idea that there was any material removed from the wood.
The "cut" is more like a "stab" in that the tool is driven down into the wood cutting the fibers as it goes and leaving a little trench.
Because no material was actually removed, the wood would like to go back to it's original position. This is why, after the ribbon is inserted into the groove and the wood is moistened it pinches the sides of the ribbon, holding it in place.
I usually use a very fine toothed triangular file to create the grooves on both sides of the ribbon.
I do this by laying the ribbon on a wooden table. Then holding the triangular file so that one of it's points is resting on the ribbon, I pull it along the length of the ribbon leaving several small grooves for the wood to "bite" on when it closes after wetting.
By the way, insert the ribbon to bottom out in the groove or be flush with the surface, whichever comes first. If some ribbon is protruding, gently file if down after the stock drys.
zonie