It is easy to understand things in slow motion videos. Check out this great slow motion video that will help you understand SLR Camera shutter system.
The single lens reflex (SLR) camera allowed the photographer to see exactly what the lens sees. In twin lens reflex and rangefinder cameras, the viewfinder image differs from that captured by the lens. What’s interesting to note in SLR system is that in the faster examples, the second curtain begins to close before the first curtain has completely traversed the shutter. This typically begins to occur around 1/250 s and this is why flash sync speed exists: because there is no time when the shutter is completely open, firing the flash at any point will cause the light to be blocked on the part of the sensor.
This video by The Slow Mo Guys will help you understand rolling shutter phenomenon of SLR cameras