The Ski Man Roto Guide is an accurate true angle with ergonomic grip, a good choice if you haven't had much experience edge sharpening skis.
A locking system that allows you to use any file or stone and locks it in to avoid unecessary wear on the guide or tool.
When using keep in mind:
Side edge beveling will give the skis more grip on the snow surface. Higher angles should be reserved for experienced racers; if the angle is too high the ski can "rail out" on a less experienced racer.
Higher angles are quicker to dull.
An aggressive edge (high angle - 86), may be more difficult for lighter racers (young kids or women) and may make it more difficult to recover from being up on the edge.