Spiral upcut bits provide a nice smooth cut in all plunge routing operations pulling waste from the cut leaving a nice flat bottomed mortise and permiting accurate cuts with no chatter. Machined from solid carbide, these production quality bits provide exceptional results for serious woodworkers.(Parker Converse)

Most woodworking gurus will tell you that for milling holes, a spiral "down cut" is preferred because the "up cut" will lift the wood on the surface possibly causing some damage. I believe this is valid for veneered wood like plywood or similar but for solid hardwood this is not likely to happen. The "up cut" leaves the hole cleaner with less debris to obstruct the cutting. Thus I believe the bit will last a good bit longer before sharpening is required. (Hal Taylor)