The Tail-Lifting Button
Some horses have very tight, clamped tails. Massaging the "tail-lifting" button, can help them relax.
It can also stimulate their anus enough to expel gas which may be helpful with colic.
Your horse should be properly prepared ahead of time before you suddenly attempt to go under his tail. Don't get into a position to be kicked.
In the first picture, this horse's tail is tightly clamped to his hind end.
Standing to the side, you can gently, carefully, work your fingers to under the tail. On the side of the anus is a point that, when massaged, helps the horse raise and relax his tail.
The second and third pictures show the horse raising his tail a little, then a little more.
The last picture shows the two center fingers massaging in the area of the "button".