I don't think this was addressed, so I thought I'd try. Understand that I am by no means an expert on revolver tactics.

If you tap up slightly on the extractor rod while the muzzle is pointed towards the ground, the unspent cartridges will slide back into the cylinders while the spent cartridges will stay out. You then just remove the spent cartridges and insert new ammo.

The brass casings expand once fired and will not slide back down into the cylinders as easily. It may just be the heat or the detonation of the gunpowder that does it, I'm not sure which one.