Equipoise was actually created while attempting to make a product which would be be a long acting injectable d-bol (Methandrostenolone). What was actually created was a product which, in the real world acts nothing like D-bol, despite its similarity to it chemically. A simple way to think of Equipoise, chemically at least, is simply as Dianabol without the 17-alpha-methyl group (thats the thing which makes D-bol able to be ingested orally and not be destroyed by your liver). However, having had first hand experience with both Equipoise as well as D-bol, I can tell you that the results from each are vastly different.
To make Equipoise, a double bond was added between carbon atoms 1 and 2 of the Steran Nucleus of Testosterone. What does this mean? Well, first of all, since Equipoise was created by one simple modification in the testosterone molecule, you could rightly suspect that it shares many similarities with it. Equipoise is just as anabolic as testosterone (as you can tell by its anabolic rating above), but only half as androgenic. Those ratings can be quite deceiving though, as I don't know anyone who would claim that you can gain as much weight on Equipoise as you can gain on an equal amount of testosterone (even though strength gains from the two compounds are very similar).
Its not very common to compare Equipoise to testosterone; however a far more common comparison is between Equipoise and Deca. I suspect this is because when Dan Duchaine introduced this compound to the steroid using community, he made an immediate comparison to Deca, speculating that it would act similarly to Deca but like a much stronger version of it. Equipoise doesn't actually act much like deca at all; Deca is actually a progestin and a 19-nor derived steroid whereas Equipoise is more closely related to testosterone (being only one double bond differ rent). Duchaine later rescinded his original statement on Equipoise and said that it was disappointing as a mass builder when compared with deca, but a far better drug than for both strength gains and vascularity. Unfortunately, the myth that Equipoises action is similar to Deca's has persisted for nearly 2 decades after he revised his opinion; this is most evident on internet message boards today, where many will advise against including both of them in a cycle because "they act the same way."
The 1-2 double bond that Equipoise has is responsible for many of its characteristics. First of all, it acts to slow aromatization (conversion into estrogen). The best estimate is that it does so at roughly half the rate of testosterone (1). This is the best number Ive found in studies. Athletes almost never report estrogenic side effects with Equipoise, even when the dose is up to a gram per week. Side effects caused by estrogen include oily skin, acne, and gynocomastia, and as I said, those are usually not found from Equipoise. Virilization (development of male sexual characteristics in women) is almost never seen with this compound, when reasonable doses are used by female athletes. This is one of the few injectable compounds which could be successfully be used by female athletes and bodybuilders, and isn't often faked.