All scarf joints serve the same purpose- to join two timbers together to span a distance greater than the dimensions of your lumber. There are many variations on the basic scarf joint, and this under squinted stop splayed scarf joint is one of them.
To what does the name of this joint refer? Well, the splay is the diagonal cut creating the surfaces to be joined. The under squinted stop is the thickness at the end of the splay, which is angled back a bit (the under squint). This little angle helps to create a stronger joint, and also adds a pleasing visual detail.