The D1 series use a flat and taper to align the chuck, and a set of big notched pins and cams to pull the chuck against the flat. The cams are called cam locks, so the whole system is often referred to as a cam-lock chuck or spindle. The cams are radial pins with an eccentric section, and are mounted in the spindle such that the look like an extra set of chuck pinions. Often, the same chuck key works both. The spindle face needs to be very accurately machined, as well as the matching chuck backplate, as both the taper and flat are supposed to mate at the same time. This essentially means zero tolerance on the fit.
The chuck is VERY securely mounted with this method, for instance a D1-6 can have 6 cam-lock pins, each nearly 1" in diameter, holding it on. Generally, D1 spindles have some of the largest through holes for their size. For instance, my Sheldon model R15-6 15" lathe with D1-6 mount has a 2.25" spindle through hole.
Jon