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From this thread:
A singing blade is an extra hollow ground blade with the blade between the belly and the spine being ground very thin. They will give a "zing" sound when they are stropped and...

Here's an old Henry Althoff razor with a belly. The photo is not great and the Althoff is not the most obvious example, but maybe it will help. It is faint but if you look closely you can see how...

It supposedly means has been hollowed an extra amount - an extra hollow is usually very thinly ground