I think I have figured it out... So you start by placing obstacles on the board, not touching each other, so there is one block on each horizontal and vertical line of the L-shaped track, kinda like Sudoku rules.
Then each turn you place another block (2x1 this time) somewhere on the board or move a block that has already been placed.
Then both players pick a direction and move both their races as far as they can until they hit something.
Is that right?