Port triggering is a configuration option on a NAT-enabled router that controls communication between internal and external host machines in an IP network.
This is because the connection can only be targeted to the IP of the NAT-router, since the internal network is hidden behind NAT.
This gives it more flexibility than static port forwarding because it is not necessary to set it up for a specific address on your network, allowing multiple clients to connect to IRC servers through the NAT-router.
Security is also gained, in the sense that the inbound port is not left open when not actively in use.
Port triggering is unsuitable for servers behind a NAT router because it relies on the local computer to make an outgoing connection before it can receive incoming ones.