I call this In-Hand Flexion, and I use the same lateral stretch practice (along with others) with my horses.
What we are essentially doing is hyperflexing the neck
, but we must understand the effects of it and know that we only need very little
of it, for no longer than ten seconds at the halt and even less at the walk. Everything must be adjusted in accordance to the horse's conformities, fitness level, and comfort zone. And it must never be forced. Everything must be achieved with softness.
If your mare is green, then I would only suggest bringing her head around as far as normal inside bend. She needs to be able to learn how to give to pressure out of her own accord, she needs the basic muscle built for lateral flexion, and she needs to be taken along at a pace where she doesn't feel pushed. Especially if she is a rescue.
But, this is all assuming that she is perfectly sound
. If she is suddenly resisting something that she is normally okay with, she may have something locked in her neck or poll. Getting her checked by a massage therapist and temporarily going back to very basic stretches would be best.