The game mechanics Google Doc for MfD can be found here.
Generally speaking, winning and losing is determined by dice rolls of one hundred each. It could be as a skill check or against rolls. The quest masters decided when it is time to roll, based on story events.
History of changesEdit
The Great Rebalancing had drastically altered some rules while buffing up every rank's stats, making it much harder for lower ranks to beat higher ranks.
At one point, the henge technique was removed as an ability because it was too hard for the Quest Masters to plan around. When this change happened, it was referenced in-universe with an Interlude chapter featuring Kagome.