I don't know what this setting actually does, but ever since upgrading my WRT54GL to Tomato 1.18, I've been having trouble downloading files over the net (noticed it with HTTP and HTTPS mostly -- possibly exclusively). I recently upgraded to 1.19, hoping that whatever was causing the problem was resolved, but it didn't seem to make a difference... that is, until I noticed this "Inbound Layer 7" setting under Advanced-->Conntrack/Netfilter. Out of curiosity, I unchecked it and sure enough all my connection problems seem to have disappeared. For some reason, when "Inbound Layer 7" is enabled, brief TCP connections like to web pages seem to work fine, but longer connections, like downloads of larger files seem to get stuck at various completion points, at which point I get a slew of duplicate ACK packets and then a "TCP Window Full" at which point the connection seems to hang (all this seen in Wireshark.) I initially suspected QoS, but disabling QoS had no effect. Finally, disabling this mysterious "Inbound Layer 7" setting seems to restore normal operation. No idea what this is, but I'm glad to have my Internet connection back again.