It's just like a wedged blind mortise in joinery. The rivet is a
sort of loose tenon.
I don't suppose that helps anyone not familiar with high end
Here's a drawing of a wedged blind mortise.
As you can see, driving home the joint forces the wedge into the
tenon spreading it into the undercuts.
When riveting you'd use only a single wedge to spread the rivet, and
you'd make a mortice in each part to be joined. The rivet is loose
and wedged into both mortises.