One of the listed features of the engine is:
"Additional Peer to Peer connections for reduced Server Bandwidth (with NAT punch-through)"

I'm curious as to how the NAT punch-through works and how to use it. I can't seem to find anything about it in the wiki, forum or tutorial.

it's in the Esenthel MMO, first client contacts server through UDP, the udp gets opened, then it connects with peers.
You just need to contact server through UDP, and then contact other clients.