One of the many exercises I get questions on is Tree on the Shortbox. Almost all of my clients inquire about its purpose. It's not an easy exercise to do, but it's absolutely needed. And, if you are doing your Elephant on the Reformer with one leg or your Single Straight Leg Pull on the Mat it's a great way to challenge what you already know. Whatever you do, do not skip it. If it is not your favorite exercise it's because you truly do need it. Focus on the length in your waist, and "climbing" your tree from your back.
As you walk back up leave your stomach pulling onto the box.
Sitting tall on your Shortbox pull one knee into your chest, and hold the back of your leg with both hands. Straighten and bend your leg one to three times. Think of this action as you would opening and closing the carriage. It's a strengthening motion, not just for choreography. Walk your hands up to your ankle, tip back, and begin to walk down your leg. In the beginning, you may only go until you are parallel to the floor. Over time you may find you can go into the backbend. As you walk back up leave your stomach pulling onto the box. The crown of your head is pulling towards your knee and your arms are reaching from your back to climb back up.
If you go into the backbend on the first one, imagine doing the roll up to come back up. Then, climb back down and on the second round reach under the frame of your Reformer and give yourself a nice stretch. Then roll back up. On the third time place your hands on the floor and circle your leg three times in each direction. Roll back up and then grab the top of your foot for an extra stretch. Repeat whichever versions you did on the other side.