It can slip, so hold onto the halyard. Also, don t let the line slip as the person is being hauled up the mast. Once the mast climber has reached his objective, cleat the halyard off. Make sure the cleat is secure and that you can undo it when the time comes. When it s time for the mast climber to come down, pay the line easily and evenly.
Nothing is more annoying than to drop a few feet, stop, drop a few more feet, stop and so on.
When done right, the person descending the mast can walk backward down the spar quite quickly. Remember to stop the descender as he nears the deck. For those on deck, stand out from under the person coming down the mast. The movement at the masthead can get violent, and tossing your cookies from aloft is guaranteed to tick off the crew on deck and the owner, who will need to launder the sails. But with some care, anybody can go aloft and come down safely.