Earlier I wrote about solving a problem making dados in turned columns by marrying a router plane and a joiner's saddle. You can read about my process HERE.
I edited together some video I shot of making the dados. I thought it might help in the understanding.
I move down the column in one direction to rough out the depth. Coming back in the other direction finishes the cut and allows me to move into the stopped end of the dado.
Ratione et Passionis