I have no idea.
Unless you have a martingale so tight that the elastic comes into action when the horse raises its' head?
But then, why have it that tight anyway?
Seems a bit redundant really.
I am sure there must be a logical reason ...
Hmmm... found this
"This elastic running martingale is for sensitive horses, as the elastic softens the action when the horse raise his head, on the other hand also prevents the horse from raising its head up completely. Thus making the ride more comfortable.
Elastic prevents the animal from putting its head up too high. Number of holes on the chest strap provides optimum adjustment to the rider."