A guillotine cutter takes a thin slice off the head. Single-blade and double-blade versions both exist. Double-blade is easier on the cap if the knives actually meet. The aim is a clean round opening that leaves most of the cap in place so the wrapper stays on.
Cut too deep and you unwrap the cigar. Cut too shy and the draw stays pinched. Look at the shoulder of the cap, not at a number of millimeters invented on the internet.
It is the default tool for parejo heads. Pointed figurados sometimes do better with a smaller bite or a v cut. Shop: cutters.