ORIGINAL: Mode One
Charlie P., Never heard of a 1/2 Immelman before, only the Immelman Turn, which is as MinnFlyer describes. The Immelman turn was an evasion maneuver. However, the fact that it put you higher then your foe was the advantage in it. WW1 pilots did a maneuver called a "Reversement" and I have never heard it describe well enough to understand it.
Lets see if I can do better with that. I
think a "vertical reversement" is a sharp pull to vertical and 1/4 roll, then a full down elevator deflection at the top as the speed is bled off and a 1/4 roll back as you descend. It's a reversing maneuver for low power aircraft with lots of elevator and not much rudder or aileron authority. Like a Chandelle but twisting to use elevator instead of rudder. Somewhere I have a photocopy of a period training poster that shows the reversement.
I always wondered how Max Immelman was able to roll his Eindecker at the top of an "Immelman Turn" at low speed after completing the climb with just wing warping.