Unlike the typical rotary cutter or jigsaw-style cutters, multi-cutters are essentially oscillating tools that go from left to right in a very narrow 2.8-degrees angle at a very high rate - one such example of an oscillating tool is the humble hairclipper.

While there might be a slightly overlap when it comes to the function, the multi-cutter is far more flexible, limited only by its choice of blade accessory. It is also far safer than an angle grinder and brings about its own set of tricks to the table as can be seen through this YouTube clip.