The Derrickman was tying off a stand of 9” drill collars while the floor crew was placing a single 8” drill collar in the mouse hole. As the 8” collar was being picked up, the rig floor hoist line was dragging upward against the front of the derrick board. The force of the line on the front of the derrick board caused the board to rise, pivoting at the hinge at the back of the board. The safety retracting lifeline cable passed under the raised board. Then air hoist line slipped off the board into the alleyway. The falling board snagged the lifeline and the Derrickman was drug across the fingers and off the board. The Derrickman struck his leg and head. The lifeline and full body harness arrested his fall just below the board. Because the Driller was on the board instructing the Derrickman about setting back the collars he was able to assist the Derrickman back to the board.