Hypothalamic/Pituitary/Testicular Axis. Basically put its the main elements in regulating natural androgen levels. When taking external androgens, the normal balance (ie axis) gets thrown out of whatck. When it is suppressed due to exogeneous adminitration of AAS your body greatly reduces or suspends the ouput of varying hormones which in the end result in the bodys creation of testosterone as well as other hormones. Thats just a basic summery and the topic itelf can be extremely deep and complicated.