I don't know that what you're describing is a hammerhead- it doesn't sound like it. A hammer head would require you to be in the vertical long enough to bleed down to 100-150 mph airspeed. It's not really a quick reversal, just a great way to come out of a rope.
I'll occasionally snap stall a 109K in such a way in the vertical that it will immediately position me face down at the enemy, this may be what you're describing.