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Here is an illustration:

AAAA           AAAA

APAA            AABA

AAAA           AAAA

AAAA           AAAA

Where A is Empty

B is Block

P is Player.

The player cannot move right because one step in the future of his right there is a block. Although in his right currently there is nothing. Since everytime the player moves in space, the player also moves one step in the future in time.