According to SynerStretch, there are three factors to consider when determining the effectiveness of a particular stretching exercise:
Ideally, a particular stretch should work only the muscles you are trying to stretch. Isolating the muscles worked by a given stretch means that you do not have to worry about having to overcome the resistance offered by more than one group of muscles. In general, the fewer muscles you try to stretch at once, the better. For example, you are better off trying to stretch one hamstring at a time than both hamstrings at once. By isolating the muscle you are stretching, you experience resistance from fewer muscle groups, which gives you greater control over the stretch and allows you to more easily change its intensity. As it turns out, the splits is not one of the best stretching exercises. Not only does it stretch several different muscle groups all at once, it also stretches them in both legs at once.
Having leverage during a stretch means having sufficient control over how intense the stretch becomes, and how fast. If you have good leverage, not only are you better able to achieve the desired intensity of the stretch, but you do not need to apply as much force to your outstretched limb in order to effectively increase the intensity of the stretch. This gives you greater control.
According to SynerStretch, the best stretches (those which are most effective) provide the greatest mechanical advantage over the stretched muscle. By using good leverage, it becomes easier to overcome the resistance of inflexible muscles (the same is true of isolation). Many stretching exercises (good and bad) can be made easier and more effective simply by adjusting them to provide greater leverage.
Although a stretch may be very effective in terms of providing the athlete with ample leverage and isolation, the potential risk of injury from performing the stretch must be taken into consideration. Once again, SynerStretch says it best: Even an exercise offering great leverage and great isolation may still be a poor choice to perform. Some exercises can simply cause too much stress to the joints (which may result in injury). They may involve rotations that strain tendons or ligaments, or put pressure on the disks of the back, or contain some other twist or turn that may cause injury to seemingly unrelated parts of the body.