Spin Equilibrium
Jodiem
The simplest explanation is given by Ron Campbel, Flying Training for the Private Pilot Licence, Instructor Manual; Spinning Long Brief, 'The Characteristics of a spin'.
To qoute: "Once the aircraft has settled into a spin the forces and the moments acting upon it will be in equilibrium and this balance of forces and moments will determine the values of angle of attack, sideslip, turn radius, rate of descent and other factors".
Hope that helps.