A crew of 5 (a supervisor, a crane operator, and 3 helpers) were shifting GRE pipe lengths for end trimming at the pipe lay down area. The supervisor had given the signal to start shifting the pipe, unaware that one of the helpers had approached the crane from the back for a water jug. The helper did not notice the rotating crane¿s counterweight turning, which began to squeeze his chest against the toolbox adjacent to the water jug. He then shouted out. Upon hearing the shout, the crane operator reacted swiftly by stopping & reversing the crane, thus avoiding a possible far worse outcome.