Full disclosure: I have never shot PPC. Never been a cop and was a kid when I watched a lot of cops shooting it.
Seems to me that when they were using the HKS speedloaders and you
need to keep the cylinder from turning slightly to assist you in deploying and unlocking the HKS to drop the rounds, a right handed shooter would discharge the revolver in his right hand, use his right thumb to unlock the cylinder, take his left hand and put two or three fingers through the cylinder opening in the frame, turn the revolver muzzle-up and use the left thumb to eject the brass. At that point, the right hand would deploy the HKS speedloader while the left hand held the revolver AND held the cylinder from turning. The cylinder must not turn if you wish to unlock the HKS speedloader.
When the spring loaded Safariland speedloader hit the scene, shooters changed their methods. For the Safariland to work properly, the cylinder should spin freely and the speedloader is simply pointed to the cylinder and the button pushed and the rounds are "flung" in to the cylinder. IIRC, shooters deployed the Safariland with their left hand.
Never having owned these, I never developed a method for using them. But I wonder if cops of the day ever carried these Safariland speedloaders? They are much taller/bulkier than the shorter HKS speedloaders. In fact, it begs the question... were PPC shooting cops practicing skills they would NOT replicate while on duty? Perhaps... but by the time I watching them shoot, they were shooting PPC revolvers on the weekend and carrying crunchentickers & wondernines on duty anyway.
Ahhh, the warm memories of my youth.