Foam Rolling decreases tightness by adding external pressure to the muscle tissue, receptors in the peripheral and central nervous system are stimulated and essentially release the tightness of the muscles. Foam rolling after training, not before, is a fantastic way to reduce soreness before it starts!
Due to delayed onset muscle soreness, foam rolling makes it easier to target those specific muscles that have been trained during the workout. Extended duration foam rolling sessions that target specific tissues for a few minutes will help stimulate the active muscle pump of the body, clear out inflammation and lymphatic pooling, and tap into the neural recovery system by reducing local tone of the tissues.
Rolling helps drive blood into local areas, which allows nutrient exchange and waste to be cleared out. All this speeds up the recovery process as a whole, and can be a powerful mechanism of recovery when routinely practiced.