Only the half-spent pattern need be tied, since this is one of the mayfly spinners that falls on its side, one wing on the surface and one in the air. [An artificial that falls] over on its side with almost every cast [...] is the most killing fly that I have ever used... — Vincent Marinaro, A Modern Dry Fly Code
Oversized wings cant the Half-Spent onto its side - one wing touches the surface while the hookpoint is dry and invisible. Air resistance collapses the large CDC wings during casting, then reopens them into the intended shape before landing.
Size 12 for mottled, veiny-winged mayflies that struggle on the water's surface before lying fully spent. Trout simply can't ignore large, vulnerable insects.
Check out the entire selection of flies available from Buck Ryan of Flies for Rivers here.