HCG acts like lh in the body meaning it stimulates the testes to product testosterone, when you are on cycle im sure you are aware your body cuts out the lh signal, causing the testes to stop functioning and atrophy, hcg stops the atrophy by keeping the testes active, meaning if you use hcg throughout the cycle when it comes to pct the testes are still active, making restarting normal test production much simpler. Some say however to use HCG in pct to kickstart the testes and get them back up to size, this is where the debate comes, keep the testes active with hcg or restart them with it.