The active ingredient is the man-made auxin (hormone) 4-Chlorophenoxyacetic acid (4-CPA) that is well known to promote fruit set commercially. Bio101--Why it works: "...Fertilization within the ovules stimulates the ovary enclosing them to begin growing into a fruit. Within a few days the ovary begins to swell. This is called the fruit set. Fruit development is under the influence of the plant hormones auxin and gibberellin. Pollen is one of the richest natural sources of auxin, so it is thought that the initial stimulus to fruit growth comes simply from the auxin to which the pistil is exposed as a result of pollination. Applying the hormone auxin or gibberellin to the ovaries of flowers can induce a fruit to develop without pollination or fertilization. This is called a parthenocarpic fruit and is, of course, seedless."