The piston racing condition is basically when powering two piston in a row facing in the same direction(like placing a piston facing in any direction and place another piston facing the same direction while the bottom part of the piston is stick with the piston head of the first piston you have placed). When you power these two pistons at the exact timing, the piston at the front (a.k.a. the latest piston you have placed) should always power. However, at sometimes, the back piston (a.k.a. the first piston you have placed) may power instead of the front piston, causing the racing condition not working.