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. Instead, 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 highly effective at increasing muscular length and reducing stiffness. It just isn’t appropriate for preparing for sports performance.