This is a very popular exercise amongst trainers and athletes for the abdominals. Obviously, with the X-Vest it becomes more difficult.
The side pillar is an exercise for the abdominal that is quite different then what we are used to. This exercise requires that you hold the position statically, which is something that most people have an incredible amount of difficulty doing. In the picture, Doc Ryan is performing them while on his elbow and knee. A more difficult way is to do it on your feet and elbow.
Start with sets of 10-20 seconds and work up from there.