First you judge CR, then you do TofC to determine if your evaluation was correct in accessing the proper control risk.
If CR/RMM is low, then you can rely on CR (Reliance on CR is high), so you can allow your DR to drop, which results in less TofD/Substantive testing.
Reliance high, CR low -> DR high, less substantive testing. Inverse relationships.
Reliance on CR is low, then CR is high, in order for AR to be low you need to minimize DR, and the only way to lower detection risk is to do more substantive testing, more test of details.
Remember AR = IR x CR x DR
And DR is the only one the auditor can control by doing more or less TofD or Substantive Testing.
If the TofC fails ie: you determine the controls are worthless, then you access CR at the max, and must do more substantive testing to ensure the errors are not making their way onto the financials.
Hope this helps, post again if you need more clarification. helpful to remember inverse relationships.
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