How it works I'm not sure, may contain an ethylene-like compound and most likely a lot of hormones derived from the kelp (cytokines, auxins, gibberelins ,etc). Since cytokines travel upward from the roots to the stems naturally, it is most likely based on that class of hormone. Zeatin is the most prominent cytokine in the plant world and most likely the prime ingredient since they are naturally derived. Gibberellins are of no use (unless you want the tallest piece of shit plant out there) and auxins tend to delay harvests while staying localized in the roots when applied to the roots (superthrive etc.). So to answer your question on HOW it does what it does, don't even bother. With hundreds of naturally occurring hormones in kelp and thousands of ratios to formulate, best just ignore the HOW.
I have used it on two of my past grows and it does work very well but be warned, more is NOT better! I like to start with a half-strength solution and work up from there. Humbolt County products are amazing but they often burn plants to death when used improperly or for no reason at all. My last crop I was over zealous and used PurpleMaxx in conjunction with Gravity without leaching the soil first. What a mess! Half of the foliage was gone in 3 days!
Best to always use it without nutrients and after you flush the soil. The company claims that Gravity (and other products) increase nutrient uptake. If this is actually whats going on or not, keep nutrient levels low when first applying, just in case!... and that goes for hydro as well.
Keep using that molasses though! I think that was the only thing that saved the plants and boy were my buds sweet!