It wasn't enough to use a 'two-two' pistol shaped like the 1911 .. because it wouldn't recoil enough to simulate live fire. The issue wasn't felt to be just getting a practice gun that worked with the cheaper rim fire ammunition .. but one that mimicked the 1911's heavy recoil and helped new shooters to get used to firing the big bore.
David Marshall ("Carbine") Williams developed the idea of a 'floating chamber where a chamber block might move back on firing - permitting propellant gases to "leak" into the gap between barrel and chamber and slam the chamber and slide rearwards .. boosting the recoil force of the slide.
The floating chamber acts much like a short stroke piston and also lead the way to his conversion of the Browning Machine guns to run on .22 LR.
Williams's floating chamber system is both blow-back and gas operated to make an accelerated blow-back.
This linked explanation & slow motion capture really make the function clear:
.. that's brilliant work eh.