To ignore the inside nic means that the rule will not
check for inside network interface.
This may be handy if you have two interfaces and one
subnet includes the other and you want the included subnet
to be able to access the IPs in the bigger network. An example:
I have a private network 192.168.1.0/24. I also have a second
NIC for my notebook 192.168.1.252/30. Now I would have to
have rules for both networks. But then the notebook would
only be able to reach my server through its IP 192.168.1.254.
But the nameserver entries all point to 192.168.1.1 but these
would not be allowed by any of the rules since the interface
is being checked (192.168.1.0/24 would match, but the interface
is wrong). So we just ignore the outside interface and everything
is fine. This makes firewalling under some circumstances a little
bit hazy and may open holes so be careful, but in some situations
(as the described one) you just need it...