I have a comment: it's a bad technique that cannot be discouraged enough.

What it does is either enlarge the faults the lens itself may show more when used at close range, and add the poor correction or the extender.
Or enlarge the poor correction of the extender and what that thing makes of the perhaps somewhat lesser performance of the lens that's on it.

In short: having the extra glass somewhere between subject and film is still not a good idea.