It's not just any old stretching.
There are two types of stretches.
[/B]Dynamic[/B]andstatic

Dynamic stretches include all exercises that involve movement. These increase you sport performance by 20% and decrease the chance of getting injured.
Static streches, include thing like touch your toes basically thing that require holding for a while. These are for warm downs and if used to warm up you have more chance of getting a injury and they decrease sport peformance by 20 percent.