Because a split sole is comprised of two separate and independent pieces of material, they permit a far greater range of foot motion than the more rigid full sole variety. While full sole slippers offer the distinct advantage of promoting muscle strength, this is only of benefit during the early stages of your ballet career. Once your feet are as strong as you need them, flexibility and ease of movement become your main priority. Landing and pushing off to leap are easier when the sole of your foot is less restricted.
While the appearance of a split sole ballet slipper is hard to distinguish from a full sole slipper when the dancer is standing, the appearance when the foot is raised is obvious. Typically the split is a slightly different color to the rest of the slipper, which creates a contrast when the foot is extended “en pointe.” The key advantage of split sole slippers is that they fit more closely to outline of the foot, making a more elegant silhouette.
Ballet slippers have elasticated tops to help keep them in place while the dancer is manipulating the shape of her foot. Because a full sole is more rigid than a split sole, it is more likely to push against the back of the slipper when the front of the foot is bent. On a split sole, this isn’t a problem as the gap between the soles acts as strain relief, permitting easier foot movement and reducing the chances that the back of the slipper will be pushed away from the heel.
The sole is the heaviest part of a ballet slipper, because the material used is thicker and more robust than the lightweight canvas or leather used for the upper portion. Therefore the less sole on the slipper, the lighter it is. Naturally, lightness is an advantage for a ballet dancer, given the requirement to move elegantly and quickly. A heavy slipper would restrict movement.