Hi Colin, what Ken made are called compound angled dovetails. They require some tricky layout. The trickiest part is cutting accurate compound angled butt joints. They can form a box with either two or all four sides splayed at the same or varying angles. A somewhat less tricky bit is how to layout the dovetails on those compound angled butt joints. The easiest part is actually cutting the dovetails once they are layed out. There are a number of explanations as to how to cut compound angled butt joints on the web, but many of them are designed to show how to lay them out for cutting on a table saw. There are a few that talk about how to cut compound angled dovetails using hand tools. For example there is one by Bob Rozaieski and another by Chad Stanton. I do not remember Paul ever doing them, but I may be mistaken.
Many web sites that talk about them make it sound as if they are (at least nowadays) quite unusual, at least for hand tool workers. But they are found in a number of traditional projects such as sea chests, baby cradles, certain sorts of serving trays, and so on. You need them whenever you want a box to have splayed sides. One last thing — It is relatively easy to layout the dovetails if only two sides are splayed and the other two are straight up and down. It is a good be harder, in my experience, to get things to work if all sides are splayed, and even harder if they are splayed at different angles!