So, basically when a convoy is raided and you have ASW, for each raiding submarine, you roll:
one D6+1 compared to the raider’s D6 to see if your convoy loses IPP one D12 against each raiding submarine, hitting on a 1 or 2The D12 is a “free Defense roll”, as it does not depend on surface warships or aircraft on MAP being on escrot duty.
See table 9-8 in the rules for all modifiers, including ASW.