Concentric threads are a shape that is expanded in size concentrically with each rendition rotated forming a thread. All regular shapes such as trianlge, square or pentagon, form a thread that has linear tensile strength. The rest of the shapes have some degree of lateral resistance called the Persson Lock. That means it pulls the sides of the hole into the thread. This is very unique type of fastening.
The example below is a 4 lob shape first displayed as four 1/4 pieces, then combined with the male component into the engage model. This shape has a Persson Lock property as shown with one of the 1/4 pieces from the front and the other from the side. The 1/4 piece is hanging in air demonstrating that it is holding the male thread.