EE WAS IN THE BOOM TRUCK BACKING UP WITH THE MOTOR TO BE UNLOADED WHILE SOMEONE WAS GUIDING HIM. THE BOOM TRUCK OPERATOR STOPPED WHEN THE CAB OF THE BOOM TRUCK CAME TO THE HANGING DRISCO PIPE. SPOTTER TOLD OPERATOR TO CONTINUE BACKING UP & TO BRING THE PIPE OVER THE CAB TO BRING THE PUMP CLOSER TO THE STOPPING. OPERATOR WAS LEANING WITH HIS HAND ON THE CAB TOP & HAND WAS CAUGHT.