The first puzzle with shirts pulled over the head is, how do you make up for the fact you're having to cut the cloth in two at the neck without leaving a big gap? One way is a placket — a clumsy look for a popover — and the other is to drag the cloth across the body in the form of a pleat, which is the method applied here.
The popover has a one-piece collar, which in point of fact, if you care to examine its underside, is actually cut in two pieces. It sweeps open at the clavicle, and rather than being hard-pressed, as the open collar on a typical holiday-style shirt, it fans out softly, and swings closely around the neck.
The popover takes every measure to make the act of actually popping it over the head as effortless as can be. To this end, it is cut in a loose and relaxed fashion, so as not to cling to the body, and has deep vents at the sides, enabling it to be opened up as much as possible before the head goes diving in.