Proportionality is when the force used is based on the extent of the threat.
In other words every situation is different. There is no ONE SIZE FITS ALL.
When someone is face down handcuffed and not moving at all and the SAME DEGREE OF FORCE is still used for minutes would you see the forcer is using proportionality or is off on a cloud of his own floating high above what's going on and getting off on his power his authority and having a real good time?
Hands in his pockets. Vapid expression. Glaring at the cameras. No emotion.
NO ATTEMPT TO SAVE THE LIFE. NONE. NOT ONE OF THEM MADE A MOVE.
In a movie you'd say it was an outrageous scenario that would never happen in real life!
You'd be wrong. DEAD wrong.