Yes, running test shuts down your own testosterone production. LH and FSH levels decease (these are pituitary hormones that signal the testes to produce testosterone) because your body senses it has testosterone and doesn't need any more produced. LH and FSH act on the Leydig cells in the testes. If the Leydig cells go unstimulated for an extended period, they COULD be less responsive to LH and FSH.
Enter HCG... HCG acts as LH and stimulates the Leydig cells so they do not become shut down. It keeps your testes working while on cycle. Then, when you come off cycle, it is much easier for your testes to rebound fully since they have been getting a stimulus throughout the cycle and it won't take them long to produce a healthy level of testosterone again.