What is it?
Turns out the diagonal threading and coarseness of denim make a naturally excellent strop for getting rid of microscopic nicks and bends in a razor. After shaving, rinse your cartridge and then, using a piece of stretched denim, push the blade backward (meaning the opposite of the direction that you'd shave) down the leg of the jean for 15 to 20 short strokes. Do it again in the opposite direction on the denim (again, the opposite way you would shave) and presto, your blade will be almost good as new.
Does it save?
There's simply no cheaper way to extend the life of a razor.
It's yet one more excuse to keep that beloved old pair of jeans around. [Free]