You just run it over the top of the envelope and it puts a nice, clean cut on the front side of the envelope about 2mm from the top

Ironically, if you don't do that you'll get a really nice cut through the envelope, but the first layer or two of the contents will also be sliced

At best, it makes a ragged, incomplete cut; what it doesn't do is open the envelope

Hasn't sliced a clean cut on 20 attempts of various envelope thicknesses, mostly regular envelope stock.