Spoofing Source IP's with a Fortigate

Something new for the blog: A Fortinet post! 

I work for a Fortinet partner, and in the last few months I've been doing more and more work with their Fortigate line.  Tonight I just discovered a really handy "feature" that has allowed me to test a change that $Provider made to their routing that I wasn't able to test directly.  Details after the jump.