I can't speak about any particular feed, but I can tell you that most "calming" feeds and supplements make use of magnesium as the main agent. The reason they work (when they do) is because most of our hays and grasses are deficient in magnesium, and magnesium deficiency effects the nerves of the horse, making them more reactive and excitable. Grass is particularly low in magnesium during periods of rapid growth, such as in the Spring -- thus leading to the "Spring fever" effect of increased spookiness, reactivity, etc. seen in some horses at that time of year.
Get your minerals properly balanced to what you are feeding, and this type of "nervousness" disappears. Of course, the only way to properly balance what you are feeding is to know what minerals are in your horse's feed sources, and that requires testing in the case of hay. I personally test my horses' hay, then balance their minerals based on that hay, which I source for the entire year. Not possible for many, I know, but the best way to do it, if you can.