Hmm,Never have flown a full scale Extra (only Pitts S-2B), but to knife edge spin any of my RC model Extra's as I pivot on the hammer head (lets say to the left with left rudder), as the nose pivots just past horizontal I leave in full left rudder and go full forward and left stick.
With these inputs the RC versions stablize quite nicely spinning about the right tip leading edge of the wing (right wing down).
Patty of course can answer directly the full scale version inputs necessary.
Dave Hamblin
(formerly DaveH on old site!)