The mainsail sheet starts near the end of the boom, or the becket of a block attached near the end of the boom. It then goes around a single block attahced to a bridle attached to the mainsail sheet attachment points on the aft transom. It then goes around a single block near the end of the main boom, through a boom sleeve or constraining loop on the main boom to a block near the centre of the main boom. It then goes to the helm's hand. Purchase is about 2.5:1
Fairly rare, but some Australian and South African boats use it. As with similar bridle systems, it allows the main boom to be brought closer to the centre line with less load on the mainsail leech than with the traditional aft mainsail sheet system. The length of the bridle determines how efficiently this system does this. A bridle which is as long as possible without going "block to block" when sailing close hauled is most efficient.
There is a 2007 advise sheet on a slightly different system, but which may help setting up the bridle.