Just tried this using the domain of a company we resell for on a phrase I've long suspected my site is being suppressed for and we jumped up 10 places. Our competitor was 9th, we were 29th......added -theirdomain.com and we became 19th.
Our site is suffering from Panda but no noticeable effects from Penguin.
To me, this looks like a duplicate/similar content suppression mechanism.
When I use amazon.com, etc. there's no difference, only when I use domains of competitors or similar sites in my niche.