Uses a roller guide. It says so in this video.

Hello Beach&BoardFan, thank you for your question. Our bevelling tools KFM 15-10 F / KFM 16-15 F are not just bevel attachments to a regular grinder. They are machines of it’s own with a different gearing especially designed and built to deliver perfect work results in weld preparation. Being a machine of it’s own kind they are way faster than an angle grinder and deliver constantly precise bevel angles for optimal welding results. With one machine one can mill all angles from 0° to 90° without changing bevel heads. This saves time and money. Last but not least our KFM bevelling tool is easy to operate because the product comes with a patented One-Touch-Controller for tool-less, fast, easy and precise depth setting. A mere bevel attachment could not provide all these features. Best regards Oliver (Team Metabo)