Port triggering is a common feature on consumer broadband routers, it is used only open ports when applications need them, but beware, these ports do not remain open permanently.
Think of port triggering as a way to dynamically (on the fly) forward ports to a PC that needs them, the PC that needs them simply performs an outbound access of that particular port range. If the PC doesn't perform this outbound access then the ports will remain closed and hence not forwarded, or 'blocked' from receiving inbound traffic by your router. If you want the ports to remain open constantly without having to use a trigger, then you'd use the "Port Range Forwarding" feature of your router. Mind you that the ports will remain open regardless, which isn't always a good idea, but if you're running a game server 24/7 then thats the only choice. This explains why when you use Firebind, it does an outbound access, the router triggers the ports to open (and forwards them back to the machine you did the Firebind test from), subsequent external connections (or scans) will then work, but will eventually time-out (trigger reset) and the ports will close.