The best made blades are forged by the most skilled swordsmiths, the metal worked with such precision and care that the edge can cut through flesh so cleanly that a person might not even know they are dead.
You have commissioned a swordsmith to craft you a Wakizashi. Before he is paid you want to be certain it is a properly made blade, and so you conduct a test. You produce a silken sheet, allowing it to descend lightly onto the Wakizashi’s edge. The sheet is cut cleanly, not a thread mismatched.