A giant whirlpool spins at tremendous speed. If you toss a rope into it, securing one end at the rim and leaving the other outside, the whirlpool will wind the rope around itself, making it spin. That's roughly how a cosmic string behaves near a rotating black hole.
In the new work, physicists considered a string that touches the black hole's horizon (its "surface," from which nothing can escape) and extends far into space. To remain "physical," the string at the horizon must rotate at the same speed as the hole itself. This causes the entire string to twist into a spiral, like thread on a spindle. As it does, part of the string falls into the hole, carrying negative energy, while positive energy is flung outward, so the hole's energy is transferred to the outside world. The process doesn't last long: the authors calculated that the extracted energy does not exceed the product of the string's tension and the hole's mass.
After a burst of energy is released, a wave runs along the string, carrying the harvested energy to a distant observer. Then everything calms down, and the string settles into a static shape similar to one previously described by other scientists. This mechanism is one way to "reach into" a black hole without violating known laws of physics.
If you could create a string with the tension of a steel cable and "cast" it onto a rotating stellar-mass black hole, the released energy would be enough to briefly ignite an entire star.
In Dan Simmons's novel "Hyperion," cosmic strings connect distant worlds, but no one suspected they could be used as batteries stealing energy from black holes.