The cylinder goes forward sealing the cartridge in the chamber.This increase the pressure although why that is a good thing in a wheel gun I don't know.
This was done to seal the gap between the cylinder and breech, ostensibly to minimize gas pressure loss. If you ever see a large caliber revolver fired, or seen one at night, you'll see fire come out of the cylinder/breech gap.
I've read that it was good for about a 50fps gain, which didn't really justify the extra complexity or the 12lb trigger pull. However, because of the gas seal, it's the only revolver for which a silencer is practical..