How to see the illusion:
Rotate your head as much as you can away from the screen and then quickly shift your gaze back onto the clock above. The more you rotate your head and the faster you shift you gaze back onto the clock, the easier it is to catch the clock in action. The clock would appear as if it is still for a moment. In other words, the exact second at which you look at the clock appears to last longer than a second. Try this a few times if you can't see the illusion right away. The illusion is easier to see with bigger physical clocks in case you have one around you.
Many people report observing this behavior even though they don't realize that it is an illusion and that it is a normal experience with a scientific explanation.
Why does this happen?
Now to the why part. As mentioned above, the key to observing the second-hand illusion is a quick shift in gaze. Although quick shift in visual attention is a routine one doesn't pay much attention to, your brain has to resort to some sophisticated gap-filling to make the process appear seamless. Basically, when you move your eyes rapidly, you cut off visual input to your brain for a brief period and instead of showing darkness for that duration, the brain provides a seamless visual experience by filling in the duration of rapid eye movement with whatever it sees after the eyes settle.
In the case of the second-hand illusion, the first thing you see after a quick shift in gaze is the clock. So, the brain fills in the time you have taken to shift your gaze (lets say a quarter second) with whatever time stamp it sees first after the eyes settle. So, the second you see immediately after shifting your attention back to the clock would appear to last for one and a quarter second giving the illusion of a still clock.
This visual input filling done by the brain is not usually noticeable and is pretty useful because it provides a continuous story of the world around you. It is only when you look at things that are designed to move with precise regularity (like a clock made by humans) that you are able to observe this.
Note that this illusion can happen with other senses as well, not just visual. Although much difficult to observe, the same illusion happens when you shift your hearing attention from one source to the other (ever observed the first ring when calling someone on phone last longer than the following rings?). Similarly with other senses as well.
Were you able to observe the second-hand illusion now or did you ever observe it before? Let us know in the comments.