The sen is a tool for stock removal and excellent for establishing flats and bevels on sword blades. It works much the same way a woodworking draw knife works, but on both the push and pull stroke. To use the tool effectively you need to find the right angle where it cuts efficiently. Too steep and it will chatter, too low and it will skate, just right and it will peel curls off the blade.
It is best to remove the scale from the blade first, but either rough grinding or stoning. It takes a while before the sen establishes itself by removing the high points then it begins to cut aggressively. It is best not to work too aggressively or excess heat will create carbide pimples on the blade and they will have to be dug out before more cutting can occur. I use the edge of a worn out file for this.
Go at 45 degrees to the original cuts to remove high spots until the surface is flat and true. With a little practice, this becomes a very efficient tool and is quite pleasant to work with. In my shop I have several in different sizes and shapes.