Why do we see this instant change? Horses choose a stance that is most comfortable to them, or rather, is least painful for them. In the before-trim photos, the horse unloads the heels by standing under and putting more weight on the toes. Once the hooves are trimmed for comfort, then the horse finds it quite pleasant to load the heels and chooses to stand with vertical cannon bones, which allows it to use it’s stay-apparatus.
These photos show the immediate impact of a physiologically correct trim on a horse’s stance. On the left is how the horse chose to stand when it was “due” for a trim. Here we see how it stands under with the front end and compensates by standing under with the backend. Immediately after a trim, the horse chose to stand “square”, aka vertical canon bones.
This illustrates the meaning when a horse is “due” for a trim. It should not be driven by a trimming interval as we set it in the calendar, but rather should be dictated by the horse’s comfort level, which we can gauge by its way of standing. If it chooses not to stand with vertical canon bones, then the horse is compensating for some discomfort in the body. This can be a number of things, or combinations thereof. However, more often than not, as shown in these cases, the hooves are a major contributor.
Update
Below an update of this horse. At the next trim, 3 weeks later, the horse is standing under again, see blue lines. I also placed the original red lines on it from the first comparison. However, the horse is not standing under quite as much. Immediatly post-trim, the horse stands square again.
Horse Stance comparison before and after trim 
This hightlights the point made above. As time progresses between trims, most horses tend to start compensating for uncomfortable feet again. A physiological trim can remove the pain sources and allow the horse to stand correctly. Trimming intervals must be chosen based on when the horse starts to compenstate, rather than an arbitary date in the calendar.
Another aspect to point out, the better the boarding conditions support the horse’s needs, the later the compensation will set in.