You can read about it on Samy's website here;

Samy Kamkar - NAT Slipstreaming
exploit NAT/firewalls to access TCP/UDP services bound on a victim machine

The repo is also hosted here on github;

samyk/slipstream
NAT Slipstreaming allows an attacker to remotely access any TCP/UDP services bound to a victim machine, bypassing the victim’s NAT/firewall, just by the victim visiting a website - samyk/slipstream