The electron beam was generated by a Van deGraaf accelerator. If you were to look through the plastic block on edge, you would find that it was clear, except at the center where all the electrons stopped. To discharge the plastic, the block was stood on it side. The metal point served as a discharge point to ground, just as a lightning rod on a house, or a tree in a field, or a key on Ben Franklin's kite, does for the charges in a cloud. All the stored electrons in the plastic drained out through the point where the metal point struck.
Secondary discharges continue for many minutes afterward.
We attempted to film the process with a very fast camera from Kodak. The film taken at 4500 frames per second is shown in this movie . We could see no more of the process when we filmed at 40,500 frames per second.