I think it's pretty obvious what it sends - or rather, the obvious stuff is enough to bother some people when the feature defaults to on.
They're saying they'd search for "Joe Random - Resignation Letter - FooCorp.doc" trying to find the latest copy they'd written and when that was sent to Amazon as a product search it would leak private information.
So I take it you've reviewed the source? Sat and watched a packet stream? The thing is I've heard lots of complaints about it infringing privacy, but not from someone who's also reviewed the source, it's all just second hand information.