A decking device for use in a hoisting system of the general type having a shaft and a conveyance supported in the shaft by a cable. The device has two jaws movable in a guide towards each other to a first position in which they engage a structure on the conveyance to immobilize the same in the shaft, and away from each other through a second position in which one of the jaws is in contact with the structure to a third position in which the jaws are retracted and can no longer contact the structure. The jaws are driven between the first and third positions at a controlled speed to eliminate any tendency for the conveyance to move uncontrollably in the shaft due to stretch in the cable.