Reading along and taking all of this together: This problem is not
specific to the Funtana, and frankly it's not really a problem. It
happens on a lot of airplanes when pulling hard and pitching up
vertically into a "wall". If the entry into a wall is smooth (not
pulling so quickly with up elevator) it does not happen at all.

So I'm pretty sure about what I wrote before that the gyroscopic force
(precession as mentioned above) causes a right yaw. If the airplane is
pushed into the stall region for just a moment, then a slight right
snap roll (1/8th roll or a bit more) will happen. It makes the wall
look a bit sloppy. If, however, the elevator pull is less aggressive,
then the airplane does not get into the stall region and the snap
thing does not happen. Instead the airplane points straight up
without the wobble.

I think most people do walls at near full speed and drop back on RPM
just before going into the wall. Whether it's full throttle or not,
the prop is still spinning so the gyroscopic force is still there.
Also, if the airplane gets into the post-stall area, this stuff shows
up (independent of the entry speed).

A couple of more things: Yes, there's prop swirl that hits the left
side of the rudder. This produces a left yaw. There's also P-factor
which causes a left yaw in a pull up. But I'm thinking the gyroscopic
force is winning the war, and then some interesting post-stall aero
happens per above.

Some ideas for experimentation:

- Do a wall more smoothly, less elevator. Problem goes away because
the plane does not go into post stall.

- Do a wall coming down the vertical line with the engine killed.
(what's this an "elevator" or "parachute" in 3D jargon?). Problem
goes away because there's no gyroscopic force from the stopped prop.

I've seen this in real life, thought about it before, and this is my
thinking on it so far. Caveat: My thinking is subject to change.

Michael