In the Butt Tuck, a pressure point on each side of the tail is triggered so that the horse lifts his barrel and tucks his pelvis. It is similar to the "cat" position of human yoga.
In the two photos above, note the change in the topline and also the change in the shadow at the hip as Apollo "tucks."
How to do the butt tuck
Press on the buttocks on both sides of the tail. The exact spot is slightly different for each horse, but is approximately 7 inches directly to the side of the top of the tail.
You will need to press firmly with your fingertips. Some people find better success with using the cap side of two pens. Finding the exact spot can be tricky, but you'll know when you do!
Ask the horse to hold the stretch for 5 to 10 seconds. You can walk him forward to release the stretch if he does not lower himself. Repeat a few times as needed.
Some horses will arch more than others, due both to their own flexibility and to the skill of the "yoga partner" (that's you).
If your horse is too tall to see over their hindquarters while pressing, have a friend help you judge if the horse is "tucking" as you learn to do this.