If you're an experienced finish carpenter, it won't matter that the subtitles in this video are in Dutch, because you'll be able to figure out exactly what this guy is up to.
The video is about the FrameFitter, a device that can be used to fit new door blanks into existing openings. At first I thought – okay, well this is a pretty fancy templating jig. It was not till later that I realized how much more there was to the device. When the timmerman (that's Dutch for carpenter) is ready to size the door, he lays the jig on it, snaps a Festool guide rail track in place, and uses a plunge cut saw to trim the edges. By referencing the rail off the FrameFitter, he was able to quickly get a near-perfect fit.
Few carpenters could justify the purchase of such a specialized device, but for the contractor who regularly retrofits doors, it could be a lifesaver. When I look at this tool I picture jobs I've been on where we had to replace a ton of doors without disturbing jambs and trim, and buildings with casing-less jamb details where we were forced to install jambs before the drywall was in. Back then I'd have killed to have this gizmo.
As far as I know, this tool is not distributed in the U.S., but I found it on a Dutch website and they wanted about $1,200 (1,000 Euros) for it. One of the guys on the Festool owner's group posted a translation of the Dutch subtitles – though as I said before, if you've hung many doors in old jambs then you can tell by looking how the FrameFitter works.
Jim Reeder of San Jose California made a door Jig. I used to have one 30 years ago. It used a "Skill 100" planer. A nice planer that is no longer made by skill. The Reeder Door Jig insured a perfect fit that was important when installing brass interlocking weatherstripping.