HCG and other drugs are often referred to as masking agents. I have no idea why, since they don’t mask anything. They’re detectable, just like the steroids that they’re often paired with. They’re a co-agent, something used to either have an additive effect (“stack”) or to counteract an unwanted effect. It’s the latter for hCG. If you take anabolic steroids, the testes shrink. Why? The body has enough testosterone — steroids convert to testosterone in the body* — and the feedback loop in the endocrine system says “shut ‘em down!” The body is the ultimate ‘use it or lose it’ machine, so when not used, things shrink away.

(* I’ll use the Posnanskian asterisk here to say yes, it’s a bit more complicated than this, but it’s good enough for this explanation.)

Men who wish to have normal sized testes while taking many anabolic steroids can take hCG. This means they will have more testosterone in their system, both artificial and natural, which can lead to dosing problems, though that’s another article for another day. 

The best analogy I have is Prozac. The popular antidepressant is very effective, but has some side effects. One of those that’s been noticed is impotence. (I’d be more depressed if that was the case!) To counteract this, one could either stop taking Prozac (bad, if depressed) or the doctor could prescribe Viagra/Cialis/etc to overcome the undesired effect.

This SAT analogy should be clear: Prozac:Cialis::Anabolic Steroids:hCG

HCG does not mask anything. It is a fertility drug in women in the same way that Viagra was initially tested as a hair re-growth drug. You could say it without being incorrect, but it’s hardly precise and misses the point entirely. HCG is like throwing a sheet over your head and hoping no one sees you. You’re likely to be seen along with the sheet, which only makes you found and looking stupid while being found.

I don’t know how to explain this any more clearly.