The guy in the bar was correct. Many old flat trackers had an "engine brake". Especially the Jawas. As some else mentioned, it was a second spark plug hole. It had a spring loaded valve attached to a a tiny lever that hung under the clutch lever. The rider would pull the lever to instantly dump all the engine compression and then Pitch the bike into the corner. It sounds very similar to a semi Jake brake. It's the coolest thing to hear and see. Takes me back and I'm not that old.
