Over here I asked about ground channel coupling because I didn’t understand how it could be useful.
All it does is tie the channel input to ground. But it doesn’t do that by shorting the channel input, rather it just substitutes the probe input for a ground input.
Apparently this used to be useful for calibrating old CRO scopes. You would set to ground channel coupling, set ground where you would like it on your cathode ray tube, then set back to either DC or AC coupling.