For normal in flight operations the Ice Detection Override knob would be left at AUTO (which will turn on both Engine and Airframe anti-icing if ice is detected). However if ice accretion is observed on windscreen/wipers and the system has not operated then the policy is to put the Override knob to ALL which turns on the anti-icing protection. (There was a case a number of years ago where the sensors did not operate in time resulting in an autopilot disconnect and a rapid roll - hence this policy).
Having said that normally the Auto system works just fine.
Slightly different procedures for ground ops and take off in icing conditions below 1500' - in this case the Ice Detection Override knob is placed to ENG thus turning on engine anti-icing protection without relying on Auto sensing. Goes back to AUTO in the after take-off checks.