Sai Mun Chui Sheut famously demanded that Luk Siu Fung shave off the latter's own mustache as the price for helping Luk deal with the Golden Roc Kingdom's enemies. We didn't get to see the shaving process, but ostensibly, Luk probably did it for himself in the normal way, using a razor blade.
What if Sai Mun Chui Sheut offered to shave you (this is for us guys, obviously) using his sword? Let's say you've got about three days' growth of facial hair, and your mouth and chin are lathered up with shaving cream. Sai Mun Chui Sheut is drawing his sword.
He's given you his word that he won't harm you, but do you trust his skill enough? Also, how smooth/clean do you expect the shave to be?