Using a wall, you are limited to being flat against the wall, thus being difficult if you have a lack of shoulder elevation or thoracic mobility. By walking backward a few steps using a suspension trainer, you can regress this and eventually progress your arms to parallel with your body.
I also like to place my hands in the foot straps and push posteriorly to activate upper quarter posterior chain during this prior to any upper body workouts. Try it and let me know what you think in the comments!
Check out The EDGE Suspension Trainer here! Just $59.99! http://goo.gl/NtcN9q