Some DA shooters will actually use the thumb to help control (stage) trigger pull with small revolvers. They pull through until the trigger finger contacts the tip of the thumb, then use that to control the final pull.
That's how I ended up having to shoot my 642.
It took some learnin', but I eventually got to where I could stage by hitting my thumb (the three chambers that actually locked up before the hammer dropped, anyway...
). But it also took some training to maintain proper grip while simultaneously relaxing part of the thumb to let the trigger ease to the break.
And for rapid fire, I trained myself to slightly relax the tip of my finger while operating the trigger with the first joint/upper portion of the second pad. It was very unnatural, and probably a small part of my desire to move on to something else.
The J-frame just doesn't work well for my hands.
Ruger LCR... not a problem.
Ruger SP-101... not a problem.
S&W J-frames... no bueno.
But the Rugers have different geometry.