Like the Bristol tackle, this is an alternative tackle for tightening ropes, from page 169 of the 1953 Scout annual. John Sweet writes:
"Another idea based on the same principle would be to make a second rope fast to another tree at right angles to the runway and then throw a loop around the main hawser and then strain it to one side liek a bostring. Crude but effective (see sketch).
So far as I can tellm the strain at X would be no more severe than if the hawser had passed over the crutch of a pair of sheer legs. If necessary, an old sack could be inserted at X to minimise the friction.
Another advantage of this tackle would be that it would enable you to set up a spanish windlass in your straining rope- a device you could not conveniently emply in the hawser itself inless it was running more or less parallel with the ground.