HCG is a peptide hormone similar to Growth Hormone and therefore can be administered under the skin with a small needle (subcutaneous), or in the muscle with a larger needle (intramuscular). Subcutaneous injections of drugs that can be used both under the skin and in the muscle differ in the speed of "onset of action". If you inject HCG subcutaneous, you will have a slower onset of action and it will last longer (longer half-life).
If we inject subcutaneous we create a more time-release effect and a unique event called a 'biphasic' effect. hCG is always going to differ in behavior when compared to the body's own endogenous luteinizing hormone (LH). The level of testosterone stimulation is much greater with hCG mainly due to the larger physical size and hydrophilicity of the hCG molecule compared to the smaller more soluble and shorter acting LH.
Our bodies tend to release natural LH (the hormone we are mimicking with HCG) every single day in small bursts. This type of hormone is called a circadian hormone meaning that it is regulated in pulses throughout a 24hr period. If we are trying to normalize our system, it makes sense to restore it using drugs in a method which allow the must "natural"/"normal" manner to do so. unfortunately hCG although very effective at wakening the leydig cells to produce testosterone, is a bit overkill.
We have to take various ancillary drugs at specific times in order to make sure that the hCG does not become extremely unproductive by destroying the testosterone to estradiol (t:e2) ratio at a critical time. and there is no more critical time than post cycle AAS when it comes to the importance of precise t:e2 regulation. if you don't already know that hCG tends to raise estradiol as well if not better than it raises testosterone - and you have never gotten GYNO during PCR, you must be 1/8th irish like me and have a little luck working in your favor. hCG is an estradiol beast. especially above specific threshold doses where lots and lots of studies on male hypogonadism and fertility back this up.
This is a very in depth topic and I can't fully dive in here and still do it justice so if you are interested in learning how to regulate this process of post cycle recovery (PCR) you will surely find my ebook guide on how to use hCG to maximize t:e2 ratios in testosterone replacement therapy (TRT) and post cycle recovery from AAS - coming very soon (probably sept. or nov. of 2011).