Just for information sake! I tried another method of placing the dime, on the barrel itself resting on the edge of the front sight and the other on the front slope leading to the groove we call a rear sight, and so far I have been able to keep it in place for a maximum of two DA pulls. Maybe, the first way I tried it was wrong (coin on the widest part of the frame over the front of the cylinder.)
I had a thought about the whole proceedure, and tried the technique again using a penny. This time it did stay in place for me during multiple DA dry firing. Maybe with more practice with a penny, or nickel then I could progress back to the dime, which is much lighter and subject to the hammers impact to a greater degree.
If you try the coin thing on a flat topped semi Auto, striker action, it is wide enough to not show very much reaction.
However, with my PT111, I was able to place the coin on top of the front site, which is flat. The dry firing caused no jumping like a hammer. In my case the coin stayed in place.
But I could see no way to keep the coin on a hammer fired revolver, or pistol after the hammer strikes with the coin over the cylinder.even though the coin was in a more stable location.
I take back the almost negative comments from my earlier post.