as I said. With the two part, the mast sees 3/2 the luff load. With a one part, the mast sees 2 times the luff load. 3/2 is 25% less than 2. That much is just math.

Now to the physics. Let us consider the simplest case: a cat boat (no shrouds) with a halyard lock, the tack of the sail tied to the deck. With a halyard lock, the headboard is mechanically locked at the truck, then the halyard is slackened or in some cases removed entirely. Would you agree that in this case, there is only one compression load on the mast, that being the luff of the sail between truck and deck?

Now look at the same boat, but with a conventional halyard cleated on deck. To tension the luff to 100 lbs, the halyard must have 100 lbs tension in it. There are now two loads upward on the deck (and downward on the mast): the luff at 100 lbs and the halyard fall at 100 lbs. So we have 2x the luff tension or 200 lbs compression on the mast.

Now consider the same boat, but with a 2 part halyard. I get a 2:1 advantage in the tackle, so to put 100 lbs tension on the luff I need only 50 lbs tension in the halyard fall. There are still only two loads upward on the deck (and downward on the mast): The luff of the sail at 100 lbs, and the halyard fall at 50 lbs. 150 lbs total. That is the "free body diagram" that represents the forces.

It isn't obvious in the 2 part case, but the tackle runs from the headboard to the truck, with only the tackle fall coming to the deck - that is key to understanding it. Suppose we used a 10 part halyard. This would need a 5 sheave block on the headboard and a 5 sheave block at the truck. The line is strung through this tackle, then the fall returns to the deck. The fall has only 10 lbs tension in it to achieve 100 lbs luff tension, and adds only 10 lbs compression to the mast. Most of the force is between the two blocks at headboard and truck: this is just the luff tension already accounted for.