The Assistant Driller (AD) onboard a jack up rig was fatally injured while performing a function test of the Casing Stabbing Board (CSB) from within the unit. The demonstration was intended to illustrate to the Toolpusher, who was present, a hand controller (ascent/decent) fault related to an apparent ‘sticking valve’. When the ascent/decent control lever was activated the CSB ascended; the Assistant Driller moved the lever through its range of positions but was unable to reverse or stop the movement of the unit. The AD attempted to exit the unit onto the fixed derrick walkway by grasping the entry bar and swinging himself feet first beneath the bar towards the walkway. The AD failed to exit the unit fully when he collided with the Toolpusher who was standing on the walkway. As the CSB continued to ascend the AD’s upper body became trapped between a cross beam of the CSB running rails and the floor of the CSB. This trapped position and subsequent force applied to the upper chest region resulted in death at the scene.