Fill the ice bomb unit with water and insert the movable plunger.  Place the cast iron rod into the holding yoke and adjust the plunger height so that it is just barely touching the rod.  NOTE: Adding or subtracting shims may be necessary to adjust the plunger height.  When assembled, place the unit into the shallow dish and pour in liquid nitrogen till the dish is almost full.  Wait several minutes and you will hear a large "snap" indicating that the water has expanded enough that the plunger has broken the cast iron rod.