My understanding is this.
"Normal" EMER ELEC CONF - IR1 powered, IR 2 & IR 3 on BATT for 5 mins then depowered and lost hence double IR "fail" on reselection of GENS so ALTN LAW to DIRECT LAW.
Switch ATT/HDG to CAPT 3 and IR 1 remains powered in the background and IR 3 remains powered. IR 2 to BATT for 5 mins then lost.
Selection on GEN LINE 1 and GEN 2 prior to LDG GR DN results in retention of NORMAL LAW as only IR 2 is lost and hence CAT 3 SINGLE - i.e. AUTOLAND.
This works in the sim. I imagine Airbus don't publicise this because the idea of EEC is to get rid of as much non essential electrical load as possible and the lawyers have would have a field day if the procedure wasn't followed and something untoward occurred.....
