You know the nursery rhyme “Pop Goes the Weasel”, and instead of saying pop you make the noise by flicking your finger out of your mouth? Now imagine that the tip of your finger is the femoral head (ball of the hip joint), and your mouth is the acetabulum (socket). That “pop” is essentially what oversplits can do to your hip joint.
Most people don’t consider what’s happening in the hip socket as the leg rises higher in an oversplit. Like a seesaw, when one end goes up, the other must go down.
As you can see in the video, the femoral head is held in place by many ligaments that create a capsule, which is not elastic, unlike muscles. If these get stretched they will not go back to their normal shape, they remain stretched and unstable. This will allow the two surfaces on the femoral head and acetabulum to slide against each other, creating friction and gradually wearing the cartilage.
Here is a picture of front splits, notice that the pelvis is tilted forwards to achieve the position. Now picture what will happen to the femoral head of each leg as it is raised off the floor. The femoral head will rotate further and apply excessive force against the ligament capsule and the acetabular labrum, which is there to stop it form going "pop"! The labrum will sustain micro tears and, due to limited blood supply, is very slow to repair. Training oversplits therefore causes constant micro-damage—even if muscles seem capable of the stretch—which can lead to injuries that are slow and difficult to rehabilitate.
Another consideration that is extremely important are the growth plates of young dancers.
These don't fully ossify (harden) until teenage years and the younger the child, the softer the bone. The femur has four growth plates (which are identified in blue on the picture below), damage to these can be irreparable and lead to deformations and leg length discrepancies. I’m seeing more young children being trained like adults, yet their bodies are not the same. Putting young dancers in oversplits places enormous pressure on the growth plate near the femoral head, where the articulation happens.
In conclusion, the truth about oversplits is that they’re over-glorified. Oversplits aren’t necessary for creating beautiful lines, and they can be career-ending before a dancer’s career has even begun.