I think it goes something like you get two knives. You place a little herb on one knife, and heat the other. When the heated knife is red, you press it to the unheated one with the herb. The heat is supposed to 'vaporize' the herb without burning it and resulting in a more intense high.

I dunno, I tried it once I all I got was a broken mason jar, a house that reeked of herb, and this t-shirt.