We know the story of black holes: These ultra-dense supernova remnants exert such a fearsome gravitational pull that they suck in everything in their neighborhood (within the "event horizon"), further fueling the black hole. But the story doesn't end with gravity. Once matter is pulled toward the black hole, it rotates around the edge and spins off some of its angular momentum before it falls in.
That's where magnetism comes in. As gasses spin in a disk on the black hole's edge, they create their own magnetic field, which ejects some of the gas at the disk's exterior away from the black hole. That ejection steals momentum from the inside of the gas disk that's closest to the black hole. The gas then slows down and falls into the dark monster.
Image: NASA/CXC/M. Weiss