The most recent info I have read suggests that the apparent short term benefits of stretching muscle length increases occurs due to the person actually learning to overcome the discomfort, and pushing further into range. This effect fits into the central tensioning neural control, eliminated in anaesthesia. As a former gymnast, I have noted that I have greater flexibility than my peers, however I need to work at it occasionally or I tighten up again and lose range (or perhaps I get softer and cannot put up the the discomfort as I age). Another fact is that collagen changes during a lifetime, becoming less elastic, which must also affect apparent flexibility / range. Regarding the splits, joint cartilage and ligament distortion also ocurrs, adding to the muscle effects.
Regards, MrPhysio