This expression comes from an exercise where the class leader asks a volunteer to do everything they can do to catch a ball he throws.
At first the leader throws easy balls, then throws them wide, or throws them strong. The volunteer moves to catch them and catches them or not.
Then the leader throws the ball behind himself… and the volunteer doesn’t move.
When questions why he didn’t move, he says: the ball was uncatchable. but the instructions were: do what you can to catch the ball… But human nature is that if we can’t see that we can catch the ball, we won’t even move.