Question: Which effect causes time to slow down near black holes?
About the article "Black hole", section "Event horizon".

Candidate passages:
A similar analysis can be used to explain Hawking radiation that causes the slow "evaporation" of black holes (although this is generally visualized as the escape of one particle from a virtual particle-antiparticle pair, the other particle having been captured by the black hole).
Constructed within the framework of quantum field theory in curved spacetime, the dynamical Casimir effect has been used to better understand acceleration radiation such as the Unruh effect. 
The defining feature of a black hole is the appearance of an event horizon—a boundary in spacetime through which matter and light can only pass inward towards the mass of the black hole.
Nothing, not even light, can escape from inside the event horizon.
The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if such an event occurred.
As predicted by general relativity, the presence of a mass deforms spacetime in such a way that the paths taken by particles bend towards the mass.
At the event horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hole.
To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole.
Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it.
At the same time, all processes on this object slow down, for a fixed outside observer, causing emitted light to appear redder and dimmer, an effect known as gravitational redshift.
Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.
On the other hand, an indestructible observer falling into a black hole does not notice any of these effects as he crosses the event horizon.
According to his own clock, which appears to him to tick normally, he crosses the event horizon after a finite time without noting any singular behaviour.
In particular, he is unable to determine exactly when he crosses it, as it is impossible to determine the location of the event horizon from local observations.
The shape of the event horizon of a black hole is always approximately spherical.
For non-rotating (static) black holes the geometry is precisely spherical, while for rotating black holes the sphere is somewhat oblate.
Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still, called the ergosphere.
This is the result of a process known as frame-dragging; general relativity predicts that any rotating mass will tend to slightly "drag" along the spacetime immediately surrounding it.
Any object near the rotating mass will tend to start moving in the direction of rotation.
For a rotating black hole, this effect becomes so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still.
The ergosphere of a black hole is bounded by the (outer) event horizon on the inside and an oblate spheroid, which coincides with the event horizon at the poles and is noticeably wider around the equator.
The outer boundary is sometimes called the "ergosurface".
Objects and radiation can escape normally from the ergosphere.
Through the Penrose process, objects can emerge from the ergosphere with more energy than they entered.
This energy is taken from the rotational energy of the black hole causing it to slow down. 
The horizon area is, assuming the weak energy condition, a non-decreasing function of time, formula_17 This "law" was superseded by Hawking's discovery that black holes radiate, which causes both the black hole's mass and the area of its horizon to decrease over time. 
Types of stroboscopes.
In its simplest mechanical form, a rotating cylinder (or bowl with a raised edge) with evenly-spaced holes or slots placed in the line of sight between the observer and the moving object.
The observer looks through the holes/slots on the near and far side at the same time, with the slots/holes moving in opposite directions.
When the holes/slots are aligned on opposite sides, the object is visible to the observer.
Alternately, a single moving hole or slot can be used with a fixed/stationary hole or slot.
The stationary hole or slot limits the light to a single viewing path and reduces glare from light passing through other parts of the moving hole/slot.
Viewing through a single line of holes/slots does not work, since the holes/slots appear to just sweep across the object without a strobe effect.
The rotational speed is adjusted so that it becomes synchronised with the movement of the observed system, which seems to slow and stop.
The illusion is caused by temporal aliasing, commonly known as the stroboscopic effect