The ring will not come off with the brush, and the reason is that it is not dirt. Below the water line in a bowl, dissolved minerals in the water come out of solution and deposit as a hard scale, layer on layer. Bleach whitens what is on top of it. It does not remove any of it, which is why the ring reappears within a day looking exactly as it did.

Hardness is the whole idea here. Vitreous china is glass fused to the clay, and it is harder than the mineral scale sitting on it. Pumice is volcanic glass foam: harder than the deposit, softer than the porcelain. Used wet, it abrades the scale away and the surface underneath is not touched, because it physically cannot be.

Which means the two rules are not optional. Soak the stone and keep the surface wet the entire time. A dry pumice stone on dry porcelain will scratch, and a scratched bowl holds scale worse than a smooth one, so the one shortcut available makes the original problem permanently worse. Wet, it crumbles slightly and leaves grey residue in the water. That residue is the stone wearing down, working as intended.

The long handle changes the job more than it sounds like it should. The ring is under the rim and around the trap, both of which are hand-in-the-bowl territory otherwise.

The limits. This is for the hard mineral ring and rust staining on vitreous china, and nothing else. Do not take it to an acrylic or fibreglass tub, a coated tub surround, chrome, or a coloured enamel fixture - all of those are softer than the stone and it will dull them in one pass. Porcelain toilet bowl, wet, and that is the list.