What exactly is Helm of Brilliance’s “a sword of flame” power?

I don’t have the exact AD&D 2nd edition Dungeon Master’s Guide quote, but there is an item, Helm of Brilliance, which, among other things, allows the wearer to turn any sword to a sword of flame.

What is “a sword of flame” here, and more importantly where in the official rule books is it described?

If it isn’t actually described anywhere, is there any less official source on what this power of the Helm actually does? I’m looking for something to show to my DM.