March 2009 issue of the Cornell Flower Bulb Research Program Research Newsletter:
Topflor and similar growth regulators (for example, Bonzi, Piccolo, Sumagic, etc.) act by reducing gibberellin synthesis within the plant. Since gibberellins are important for cell growth (elongation), using an “anti-gibberellin” caused cells to stay shorter…thus the entire plant stays shorter. The positive effects of PGRs, in addition to producing shorter, more compact plants, often include darker green color, “tougher” plants, reduced water requirement (sometimes due to smaller or more “stacked” leaves on crops such as lily).