I believe You assess control risk base on your understanding of the internal control structure. If control risk is low, then I believe the controls are operating effectively, so I'll test controls and perform a limited amount of substantive procedures. If I assess control risk to be high, I will not bother testing controls, and I'll just skip to substantive procedures. Low control risk means less substantive testing, (assuming the assessed level is corrrect). Because I believe the controls to operate effectively, I believe they will catch errors, so less substantive testing by me needs to be done, which means my detection risk can be high because the company's control risk is low, so I maintain the same Audit risk.
FAR - 93
REG - 87
BEC - 84!!!!
AUD - 99!!!!!! CPA exam complete.