How is a “trigger port” used in router?

I am configuring a Belkin router in my SOHO for video conferencing. There is an area for “Special Applications Settings” that has following fields > TriggerPort TriggerType(TCP/UDP) PublicPort PortType(TCP/UDP) EnableCheckbox < . My application documents various ports that need to be open but it is not clear what is to be used for TriggerPort. Router manual examples 1 TriggerPort with many corresponding PublicPorts. I need to understand how an application uses TriggerPorts to communicate. Once I understand this, I am hopeful I will be able to read between lines in application documentation to understand what I need to setup my router. Thanks. >>Anjan>