I've heard crazy weird stories about molasses but here is the basics -
First off you want black strap molasses - this is NOT the stuff sold in supermarkets as blackstrap (although it will work to a lesser degree). True black strap molasses is gross, you would never ever eat it. Its what is left over when making sugar and store quality molasses and generally has much much much less sugars as they've all been stripped out and much much more of the good stuff - what makes it bitter. True blackstrap can be found at ag and feed stores for really cheap. My 5 gallon bucket cost me $20 which comes to like $0.50 a pound.
Ok, now what it does - Molasses has many micros, vitamins, metals and other trace elements that your plant can absorb. More importantly these micros and vitamins help encourage healthy beneficial in the soil - the same bacteria and fungi that mineralize your organic amendments making them available to your plants. Secondly, the sugars that are left in the blackstrap are a readily available form of carbon for which the beneficials eat. Bennies, although they turn N,P,K material into ionized forms, mostly eat available carbon - for every gram of nitrogen they need they must have 30-50 grams of carbon...
So in essence molasses does fairly little for the plant itself but does wonders for your soil which in turn will help your plant. I recommend feeding your soil molasses all through the grow - the notion that it's only beneficial at the ends is a bit silly imo, and the stories of plants directly absorbing sugar to make the plant sweet is a bit crazy... Ever burn sugar? It's gross - I don't know why anyone would want extra sugar in their buds.
tsp/gallon is plenty if you are using it frequently.
