Okay, I've worked out the "trick" to it.
It's not just using the left hand like in the creator's video: (https://www.youtube.com/watch?v=DKNHfcc6RlE).
That doesn't really factor in.
You angle the thing against both the rear of the loading port and the front of the trigger guard, use your index or middle finger to keep the shell from popping out, then push.
I'm becoming more consistent with it. when it works, it really is fast. Much faster than doing 3 by hand, but still not faster than popping just 1 in.
Gonna see if I can't reach that point with it.
(I feel so genteel with this.)

*enters the /brit/ dimension*