The release of the Atlas is one of the first steps in an equine massage.
If you are a dressage rider who has spent any time studying many of the classical masters, you may know about the flexion of the jaw. My late trainer Jean Claude Racinet devoted his life to the study of lightness and the work of Francois Baucher. This is a helpful excerpt of an article from JCR on flexion of the jaw and its purpose and origin. Net, it is a tool riders can employ to relax the whole of the horse.
Similarly, using pressure on the atlas points behind the ear at the start of a massage session can often help create an initial sense of relaxation that can set the tone for releases needed through the rest of the session.
As you can see in this video, despite being distracted, the atlas release overwhelmingly relaxes the horse, softening eyes, lowering the head and then active attempts to release the poll tension in the TMJ joint.
Reactions differ. Any lowering of the head or licking and chewing is a good sign.
As you get your horse worked on by a massage therapist, you should note however if there is chronic poll tension. This may be where you want to have a conversation with your trainer or your equine dentist to see if there are other underlying issues to be addressed.