Essentially, when spinning a black hole doesn't have a point at it's center but a ring (a point can't rotate).
Theoretically a black hole can spin faster than the speed of light since the event horizon is just space, not matter, and space can certainly exceed the speed of light (see expansion of the universe).
IIRC from relevant literature, the event horizon shrinks with increasing speed until it vanishes once the speed of light is exceeded and leaves behind a naked singularity; a black hole with no event horizon.
What is a black hole with no event horizon? Does that mean that once crossing, you are instantly subject to a great gravity? So there's a gravity gradient? How is that possible?
It's a naked singularity. Light can escape from the center ring of the singularity and you could possibly observe it. What exactly it is we don't know, our physics just break down that deep into the gravity well. It could just be a barely visible ring that saws your spaceship in half if you fly through or a wormhole or possibly anything really.
The event horizon is not a shield from gravity. It's the theoretical shell/radius inside which light cannot escape no matter what direction it's travelling. Photons sort of accumulate around the event horizon of a spinning black hole[0], because it takes them more orbits to escape the closer they get to the horizon, but without any matter/photos to observe, the event horizon is just a theoretical shell/radius limiting observation.
A black hole without an event horizon is a naked singularity[1], a concept in theoretical physics resulting from looking at the mathematical equations that describe black holes and observing that the event horizon's radius becomes undefined at certain angular velocities[2]. It's not clear if it can exist in reality.
AFAIK (not a physicist), gravitational force of a specific object (as a function of distance) cannot be discontinuous, can it? It's definitely not under the classical F=G(M1)(M2)/r^2 equation. Force can't be discontinuous unless mass changes discontinuously or space changes discontinuously. But I don't know if there are extreme conditions (and a naked singularity might qualify) where that equation breaks down, or is thought to break down?
It means that the black hole is close to extremal. This means that the space near them is deformed in even more interesting ways than around a boring old non-spinning spherically-symmetrical hole.
I think it's often assumed that most black holes in the universe would have relatively low spin, compared to their mass. The research here is saying that, perhaps, the way that stars collapse would produce near-extremal holes in most cases, so they would be common in the universe.
I recommend reading up on black hole geometry ad Kerr(-Newman) holes; it's likely that any description that fits in a HN comment will leave you knowing less than when you started.
I am confused by this as well. The only thing I can imagine is "the matter falling into the hole at the event horizon moves at near the speed of light, relative to the center of the black hole". But I'm not 100% sure.
The angular momentum inside the black hole should then also reflect this, but I don't know if physicists are too confident about what the matter inside a black hole looks like.
But this appears to be more complicated:
https://physics.stackexchange.com/questions/183832/typical-r...
https://physics.stackexchange.com/questions/174247/is-there-...
Taken from one of the stackexchange posts, this is quite interesting: http://www.nature.com/news/spin-rate-of-black-holes-pinned-d...