An accelerated spin in a Pitts is extremely steep. Perhaps the 89-91 is a bit of an exaggeration but whether upright or inverted it does look vertical.
To add to HappyJack's post - Bill limits that teaching to the Pitts - he calls it FART - refer his article in Sport Aerobatics magazine. He certainly demonstrated it satisfactorily to me. If I was disoriented in a Pitts I'd personally use that rather than Beggs-Mueller now.
(shouldn't we be in a different forum for this discussion the way its going?)