How works the ability "Tunnel rat" of Mezaris/Exiled?

I don't understand this ability

My first idea is to pass mountain, example we have a cliff and with this abilty we ascend the cliff but is not it.

Help me

Tunnel rat simply allows your city to continue functioning normally and producing the same FIDSI even if under  siege, which otherwise would greatly reduce FIDSI output.

Thanks, endless legend friend