This demonstration--termed water cannon by its inventor-- is a classic example of what can happen when steam becomes "entrapped" by subcooled condensate. Steam at just over atmospheric pressure enters the top of the lexon tube and condenses as it exits the bottom into subcooled water.
Occassionally, however, a steam bubble does not exit the tube, but condenses so rapidly that it exceeds the rate of the steam supply (which is choked by the hand valve).
As the steam volume (with a specific volume of 27 c.f./#) collapses into tiny condensate droplets with a specific volume of 1/1600th that of the steam, the vaccuum created in the tube draws up water at a rate of about 1 foot in 1/30 of a second (one film frame) crashing into the partially closed valve at the top. In a steel pipe, the pressure created by the impact would be about 1350 psi.
Keep in mind, the motive force pushing the water column into the void left by the condensing steam is just the atmospheric pressure pushing on the surface of the water vessel. In a closed steam or condensate system, the motive force would be the pressure of the system, i.e. much, much greater.