This is a really clear example of the processes a horse may go through when changing from a chronically stressed to a more relaxed state.
There may be other indicators, but these are some of the most significant
First...concentrating on himself. No attention paid to the environment as he focuses on himself. This seems similar to the "inward-looking" state of horses suffering severe pain
Second..jaw mobilisation. You can see with this horse there is almost a battle between his stiff, jerky, twitching nostrils/upper lip and his "trying to relax and move" tongue and lower jaw.
Third...Once the jaw has been able to relax, it seems that sensation is returning to his face which results in him needing to rub his head.
Fourth... he shakes out the tension in his neck
Fifth....... A big breath/sigh
Sixth... his ears prick, his eyes "look outward" and he engages with his environment (sniffing the floor, looking at something in the distance.)
However, this horse is still showing signs of unrelieved stress in his abnormally low head position, but for a first attempt at "letting go" he has done really well.
With practice, he will learn to regulate his emotions with more efficiency, until he becomes resilient enough to handle life without worrying so much.
The Dance
"If you can't lead them, I don't know how the hell you think you're gonna ride them"
A short clip from Professor David Marlin's presentation on Equine Respiratory Health at last week's Horsetech Conference