Let's assume we are watching 10 seconds of a basketball game. It is right at the moment when two players are standing near the basket and the ball is passed from one player to the other who reside under the basket. This player after, grabbing the ball, turns about his waist and shoots the ball into the basket such that the ball enters into the net from its below. We no longer see the game.
Now consider time goes backwards. The ball enters the basket from above, one player grabs it and passes it to the farther player. Consider the way how the players act is such that we cannot distinguish the difference between the first scenario and the second one only by watching their action.
It seems in the lack of the rest of the play, we cannot distinguish between these two degenerate scenarios. However, there is a tiny difference between the two that breaks the degeneracy. In fact, by noticing that we can distinguish between the correct and the backward time directions. Guess what is the point?
Mohammad H. Ansari