When using ICS wizard ...

[Dolby]

... what is it actually doing? If one wanted, is there a manual way of doing exactly what the sharing wizard is doing?

 

[sn3rd]

In short, yes. The wizard basically sets up routes and permissions. The "device" will have a default gateway of the next hop on the WAN port, and will have a permissions table detailing whose traffic to route (using port/network address translation) aswell as an address translation table