How often to do flexions?
In answer to the person above who asked this:
I do flexions from the ground (and later from the saddle) when first asking the horse to give me the balance I want. Usually, the relaxation of jaw and poll comes very quickly as a conditioned response and I need do it only several times at the beginning of a ride.
Should he go against my hand (throw a "contraction") in the course of the ride I might halt and refresh his memory as to the correct response, while eliminating this "brace" or "contraction."
When I get on a horse I've not ridden before, a catch-ride say or a student's horse, I spend perhaps 5 minutes doing fixed-hand suppling under saddle just to make sure the "buttons" are installed and working; not unlike an aviator going through a pre-flight checklist in an airplane.
Flexions can easily be overdone and unless the horse is throwing you constant contractions and resistances should not be done every day.
Hope this helps!