Function:

Snapback like feature ( http://www.apple.com/safari/ )

With the press of one button the user can regress through the history to a
point of origin.
where "origin" is defined as when the user used a bookmark or the last
entry of an adress into the location bar.

I Have attached a screenshot of what it could look like (icon is from the apple
one so... well I can do a better gnome-ish one though. but it servers its
purpose in this exercise)

the icon should be directly placed behind the location bar, imo it gives an
incentive to want to click it too see what it does but not without reading
about what it does first.
And it gives a unifom approach to what it is meanth to do.
i.e in this case regress the browsers history to the point of "origin"