Went out to my hay bales for a little practice this morning. First few shots (shooting one at a time, same arrow) were wandering, and kicking a bit, just weren't flying well.
Nocked it cock feather in just to see if it made a difference… and it flew just fine, hit pretty close to where I was looking.
Alternated shots, one cock feather out, next in, for the next several arrows. None shot well, or were close to where I was aiming - missing by around 4-6" at 15-18 yards. Every shot with cock feather in hit my 5" plate, at the same distance, with good flight.
I've played around with it before, never noticed such a clear difference though.