For a while I have been curious about halo, and how it works(I am curently trying BD's IP halo). I found a pretty good article that explains a bit about it so I thought I would post it here:

Halotestin also has a volumizing effect on the physique, and for those with low a body fat percentage, this will cause an immediately more contest ready appearance. This is due, at least in part, to Halos ability to increase mean hematocrit with and hemoglobin level as well as red cell mass (4)(5)(6). Halotestin also appears to act through cells already committed to respond to erythropoietin (11), which is good news for athletes, of course. As you can see, Halo has quite a profound effect on red blood cell production, and this action is clearly one of the most obvious mechanisms by which it is thought to exert its effects with regards to increasing strength and energy levels. It also points to the possibility of using it for athletics and sports where a high VO2 max is needed, such as Rugby, Mixed Martial Arts, etc..

Halotestin also exerts its effects on strength and fat loss by both regulation of fatty acid oxidation in the liver and fast-twitch muscle mitochondria (2). Oddly, for a drug which exerts such a nice anabolic effect, and promotes such good strength gains, Halotestin has a pretty low Androgen Receptor Binding affinity (14).. I suppose, in this respect it can be compared to Winstrol (Stanozolol).

As far as strength and agression goes, Halo is a great drug. Halotestin is especially useful on a cutting or strength cycle. Its use for mass and weight gains have been pretty disappointing for most users, however.

Fluoxymesterone administration is (unfortunately) accompanied by a reduction in thyroid binding globulin which causes associated decreases in T3, while the free T4 index remained totally unaltered; thus implying that thyroid function was unchanged. Remember, many anabolic steroids (notably Trenbolone) lower your T3 levels. In addition, during fluoxymesterone administration, there was a reduction in testosterone, gonadotropins and LH response to LHRH. Basal TSH did not vary, but there was a reduction in the peak and integrated TSH response to TRH. PRL levels tend to remain unchanged during fluoxymesterone use (8). Halo is of course suppressive to your HPTA, but Ive found that in some studies where measurements were made of serum FSH, LH, testosterone, up to 20mgs per day of Halo did not suppress them measurably (9). This could possibly indicate the use of up to 20mgs/day of Halotestin without being in any great danger of suppressing endogenous hormones.

Anyway, Halotestin is a testosterone derived steroid, and has an 11-beta group attached to it to inhibit aromatization, although it is particularly prone to being 5-alpha-reduced and may thus cause DHT related side effects, such as acne and hair loss. It is metabolized primarily by 6 beta-hydroxylation, 4-ene-reduction, 3-keto-reduction, and 11-hydroxy-oxidation. We know this by the identification of 4 particular metabolites and the tentative identification of at least 3 other metabolites. Detection of Halo in urine is possible for at least 5 days after a single 10 mg oral dose to previously untreated adult males, by monitoring the presence of 2 metabolites, since the parent drug is not detectable more than 1 day after the dose(12). However, the moral-compass of the athletic world, the IOC, has developed a test for fluoxymesterone metabolites that will detect them for up to 2 months after cessation of use.