Looking at using HCG before and during HCG gives me a couple thoughts.
Before:
You stop injecting test or are running prop, either way your test levels are falling. You run HCG which mimicks LH, and your Leydig cells start to produce test. There is not much test produced from the HCG. Skinnyguy180 posted about this before
here. There is about a TRT dose that is coming from HCG (Thanks for sharing that info awhile back btw). The extra test that is produced will eventually subside as HCG is discontinued (unestered btw, so the half life is extremely quick). That is important to keep in mind as it separates both methods. So you wait for the HCG to clear (Which can be roughly 5 days, but a little play either way is fine. The half life is just a little shorter than prop) which now has your testicles ready to function. Clomid is pretty fast acting. You are now running your PCT. I will just focus on Clomid for this discussion. The Clomid is blocking the estrogen in the pituitary. Estrogen controls the gonadotropins (LH/FSH) with a more pronounced effect in the pituitary as part of the negative feedback loop. GnRH is still needed to produce the gondaotropins, which is produced in the hypothalamus. Test works in the negative feedback loop by inhibiting GnRH production.
So your balls are primed and waiting for more signal to do something. Your test levels are down and your receptors in the hypothalamus are not being told to stop production, so now it is ready to produce GnRH and tell the pituitary to work. The Clomid prevents binding of the estrogen to the pituitary, so it is not dictating how much LH to produce. The pathways are clear. There is a couple days without LH simulation to the Leydig cells in the testicles using this method (They sit idle), but I don't see that as being a big deal. Maybe it is. I haven't read anything that explicitly says otherwise.
During:
You have finished all injectables and waited the window to start PCT. You inject HCG and are also using Clomid. Your test levels are low enough before running the PCT that they don't restrict your hypothalamus from producing GnRH, so that pathway is open. The Clomid is about to work to block the estrogen in the pituitary. The HCG will be used to prime the testicles. Once the HCG comes into play, the TRT dose of test it produces is enough to signal the hypothalamus to stop producing GnRH. Your pituitary is not being restricted in terms of LH, but it needs GnRH to work. You have to wait for the HCG to clear and test to subside for the hypothalamus to not be inhibited. The HCG finally clears. Your HPTA is now functioning as it did in the before example.
HCG is used solely to get your testicles ready to fire. I honestly don't see what it is doing in the scheme of things to be necessary in the PCT. It produces test, which shuts down your HPTA. That prevents the Clomid from really accomplishing anything. The one thing I do see, is that everything is ready to go one step after another. As soon as the HCG clears, your body will produce the gonadotropins as soon as possible. The
Before method leaves the timing up to you. You might slow down the process by waiting too long after HCG application to start the Clomid, when it is already built up and primed in the
During method. That is the only real difference I can see. I wouldn't say either way is wrong, just different ways to accomplish a common goal.