This is what works in my Pitts S1 for an upright flat spin.
From level flight with power at idle, slow to stall speed, 55-60 knots. When the nose drops apply full Left rudder (it must be that way with a right-turning Lycoming engine) and pull the stick all the way back.
We are now in a normal upright spin to the left. After about one turn, apply and hold full right aileron. This flattens the spin a fair bit. Then open the throttle, the more power, the flatter the spin. The yaw force on the propeller disc is precessed 90 degrees in the direction of rotation to become a nose up force.
With full throttle, the nose comes up to the horizon and there remains about 20 deg of left bank. The airspeed indicates zero, the rotation rate is less than a normal spin and the rate of descent is a lot less.
This spin in my aeroplane is smooth and stable, like sitting in a rotating armchair with engine noise.
Things on the horizon are whizzing past quickly when looking out and I mainly watch the altimeter.
Unlike some other spins that you can do in the Pitts this one is not heart-thumpingly exciting, it is almost relaxing. But the altimeter is unwinding….
For the recovery I follow the procedure from a textbook written by a highly experienced aerobatic competitor, instructor and judge.
Leaving the power on, apply full Right (anti-spin) rudder and hold it there. Then make sure you start with the stick in the right rear corner. Then while holding full back stick apply full left aileron. Then holding full left aileron start moving the stick forward. You make an ‘L’ shape. The S1 stops rotating very quickly after that, the nose goes down at the same time and you are flying again. Centralise controls and recover from the dive.
The same technique works with power off but it will take longer. This was Eric Muller’s method.
The Beggs-Muller Power Off, Hands Off, Opposite Rudder ‘Emergency Spin Recovery’ technique also works in the Pitts, but the above power on method is the quickest.