The Hoist was being prepared for the rig move. The mast was scoped in, travelling block and monkey board were secured. After having everyone positioned in accordance with the responsibilities allocated during the pre-task safety meeting, Driller started lowering the mast onto the headrest. While doing so, the top small hydraulic cylinder did not retract and the large bottom cylinder came out of the mast raising mechanism housing causing the mast to free fall onto the headrest