Stretching the hip flexors to help you run faster and get quicker has been well-established. Greater hip joint flexibility allows you to have a longer stride and a greater range of motion over which to drive the knee forward from a behind-the-back thigh position. It also gives you a greater reach. As a result, this key action can increase your performance significantly.
However, if you watch players doing a hip flexor stretch, you will see many variants, some effective, some not very effective. For example, some players assume a semi-lunge stance and bend the back leg’s knee. However, the more you bend the knee as you go into the squat, the less of a stretch there is on the hip flexors!
Do not tilt the pelvis forward or lean forward to flatten the lower back thinking that this will stretch the front of the hip. Tilting the pelvis forward with the shoulders above the hips creates an arch in the back which occurs in the running stride, first step and forward reach. But it also gives more slack to the hip flexors so that you have to go in to a more extreme position to get an effective stretch. This requires that you do the classic long or full lunge.
To do the full classic lunge, not the typical split squat that many players call a lunge, you take a long stride and keep the rear leg straight but relaxed as you go into the lunge position. If you keep the trunk erect it produces a strong active stretch of the hip flexors. To return to the standing position push off with forward leg and take several short steps backward. Do not lunge with a step backward. Always step forward as occurs in typical basketball play.
The classic lunge, as once used in the sport of weightlifting, is also great for balance (especially with a weight held overhead) and almost duplicates the airborne position when sprinting or reaching. Thus, this stretch has a very close relationship to improving your performance. For more details, see Explosive Running.