Tight hamstrings limit your mobility. This self-myofascial release technique will increase your range of motion without causing you to lose strength the way too much traditional stretching could do.
The foam roller isn't precise enough to effectively work the hamstrings. Unlike other muscles where the individual heads are closer together, the hamstrings run farther apart along the femur. The lacrosse ball, however, can get in between each hamstring head that's missed by the roller. (A tennis ball will work for this too, but a hard lacrosse ball will really do the trick.)
Use the ball on any hard chair. This allows you to accommodate flexibility while imposing the most bodyweight on one particular spot. The best ways to use the ball are: