Below Ground Grafting

A while ago, I bought a grafted Sulcrorebutia Rauchii - really great purple colouring to it too. At the time the guy said that it was grafted below ground. I never gave it a thought at the time, but now I'm curious by what he meant. Did he mean that it was a short graft, barely at soil level. Did he mean that it is grafted and the whole graft is now below soil.

I want to graft some of the pups, and wondering which is the best way to go.

I say just graft the pups onto the usual type of stock like Myrtillocactus geometrizans and just graft them normally.

The plant in question though, I suspect, may have been a hypocotyl graft. In which case the union could very well be below soil level.