To sharpen the Tonic blade, I do as the others have said but I also push the blade slightly in toward the cutter body. When you do this, the 2 metal parts work to sharpen the cutting blade. At least this is what I was told when I called customer service.
Love my guillotine cutters. I have the Tonic (got it online from JoAnn on sale) and my 9" Fiskars (got it with a coupon-this is the only Fiskars cutter I think is worth having. Over time have bought 4 Fiskars and this is the only one I have out.)