I would think that
The line would be dead-ended on the boom, go up to the leech reef cringle then down to the sheave in the aft end of the boom, through the boom and exit out near the mast end to whatever cleat, pulley etc. is there.I will say that I have seen this sort of system in use and without some way on providing some downward tension along with the aft tension the sail will not be pulled down to the boom far enough to provide a nice flat sail. I would think there would have to be some sort of turning block on the boom placed aft of where the reef cringle will eventually be when reefed. With this there will be tension on the cringle both down and aft.My reefing is external. I placed a cheek block on the boom about 4 in or so aft of where the reef cringle eventually sits when reefed. The line runs from a dead eye on the starboard side of the boom, up to the cringle, down to the cheek block and forward to a cleat. Super simple...