As with everything, there are many interpretations when it comes to the terminology associated with horses. I try to be clear and precise in the words that I'm offering, but there still can be a gray area in the human student's understanding. This often comes from their level of awareness, background, and unintentional anticipation/expectation of their mind "getting ahead" of wherever they are currently at with their horse.

I thought I'd share my perspective on a release with a horse. Here are a few of my thoughts:

What one horse experiences as a release, could be pressure or stress-inducing to another.

Often what humans assume is a release of pressure (whether physical or spatial) is not perceived by the horse in the same way and defeats the purpose.

The release is not about "asking nothing," but is a time for the horse to mentally process and emotionally purge any potentially defensive emotions. But if he is "left" standing with tension, and concern, or is distracted while offered a release, it will be containment for the horse, rather than a processing opportunity.

There is no set "time" for the length of the release, it needs to be appropriate for the particular scenario and individual horse.
Often the human is assuming the initial release is enough, rather than assessing the horse during/after the release to recognize if the horse needed more time, support, etc. to truly let down and process.

If the horse is dramatically/quickly "rushing" out of a release moment, then it wasn't a release.

If the horse is unable to continually release throughout the human interaction, probably too much, too fast has been imposed upon him, and can lead to mentally overwhelming and physically unreasonable behaviors.

The release should not be a patterned experienced, it should be offered whenever necessary, which means the human must stay mentally aware to recognize when they need to offer one to the horse.

The release should not happen only "after" a troubling or trying moment, it can still be offered when the horse is doing well.

The release is an opportunity for human assessment, and to learn how "much" the horse can let down, depending on his initial starting place.

The release will "evolve" as the horse's willingness, try and trust does. So initially it may be having the horse come to a full halt, allowing him to process, whereas later it might be offering a long rein after a specific physical movement.

The release is a GREAT time for the human to check in with themselves to notice if they are breathing consistently, if there is tension in their body, if they are standing/sitting in a balanced position and if they can slow their brain down to assess the horse's focus, muscles, posture, and breathing.