The best way to avoid S-cracks is to gently compress the floor of a pot several times during throwing and to keep excess water or slurry from sitting in the pot during and directly after throwing.
Why does the clay crack in this serpentine pattern? It is due to the way pressure is applied to the clay during the forming of the pot's floor. Uneven pressure, or not enough pressure evenly applied, allows the clay platelets to remain disorganized within the fabric of the clay. This, in turn, means that shrinkage will be uneven. The differences in shrinkage follow the differences of compression, or the patterns of the fingers as they formed the floor. Therefore, the crack changes direction as it goes through the center of the floor.