Here's the back of the block. The seams in the black pinwheel block should be pressed clockwise. (As you can see in my photo, I did not press all my seams correctly.) The seams in the red pinwheel block should be pressed counter-clockwise. If you carefully pick out the threads in the seam allowance in the center, a tiny pinwheel is formed. (You can see it better in the black pinwheel block.) Where the rows are joined together, the seams are pressed in opposite directions to help the seams "nestle" together and line up accurately.
Click here to see the front of the block.