Took me a minute to wrap my head around it, that explanation isn't worded that clearly, but then I got it.
That happens because time is a factor in how light from different parts of the object will reach the observer. Light from its far side takes longer and in that time the object continues to move. You can see behind the object, because its rear end moves out of the way of the light coming from itself during the travel time of that light.
How does the rear end move out of the way? Wouldn't it be blocking the light? It's not like the object could move out of the way faster than the speed the light it traveling at (ftl)
That happens because time is a factor in how light from different parts of the object will reach the observer. Light from its far side takes longer and in that time the object continues to move. You can see behind the object, because its rear end moves out of the way of the light coming from itself during the travel time of that light.