The manna is a great prehab exercise because it works the rear delts, and forces the often tight shoulders into a great range of active flexibility. Rear delts balance out the often imbalanced shoulder girdle. Active flexibility is, by many strength coaches, considered to be a much more important aspect of injury prevention than static flexibility.
Now another reason that I came across, perhaps most already knew this, is the role of the long tricep muscles to posteriorly stabilize the shoulder girdle, specifically in overhead movements (handstand work). Anyone trying manna work should be well aware of the massive cramps that result in the long head of the tricep.
Just another cool reason why manna is an awesome prehab exercise and I am always looking for ways to keep my shoulders injury free. Anyone close to achieving or has achieved the manna?