From a review on a Linksys router:
"Anyway, this is an interesting addition. Basically, it's a conditional port forwarding feature. If the router sees outbound traffic in the port triggering range, then the incoming port range will be forwarded to the IP of the computer initiating the traffic. This is supposed to increase the support for some online games (I only play Half-Life/TFC online myself, so I don't know which games would require this feature)."
So you specify a port number, for example 80, and when the router sees packets going out of it from port 80, it records the LAN-side IP that it came from. When the router has packets coming in to port 80, it will forward them to the original IP address that it remembers last sent them out port 80.
I guess you can say it's a sort of port forwarding.