Research over the last 10 years has shown that slow, static stretching before sport is NOT effective at reducing soft tissue injury. The slower movements prepare the wrong nerve pathways for the fast ballistic movements used for sport, and the potential for injury from the stretches themselves is considerable. Some studies have shown that lengthening a muscle before sports performance can alter its optimal length-tension relationship, meaning reduced power output during sport. Contemporary Sports Science suggests that we use faster dynamic flexibility exercises after a lengthy, progressive warm up.
This does not mean that static stretches are useless. If you have muscular tightness, regular slow stretching (two to three times per day for 15-20 seconds at a time) is effective improving range of movement. The mechanism behind this is likely to be that stretching past the painful point gradually convinces the brain that it is safe to move there, and so it reduces the protective muscular tightness allowing more range.