Let $X$ be some variety over $\mathbb{Q}$, and let $\hat{\pi_1}(X\times_{\mathbb{Q}}\mathbb{C},x)$ denote its (topological) fundamental group. As is well known $Gal(\mathbb{Q})$ acts on this fundamental group. I was browsing old MSRI videos, and in the middle of one of them I saw an intriguing explicit description of this action:

http://www.msri.org/realvideo/ln/msri/1999/vonneumann/schneps/1/main/08.html

(you don't have to know anything from earlier in the talk to understand that page)

As it says there, there was also a talk by Ihara about this. I'm looking, however, for an explanation of this in a more systematic way, in a paper or a book. Do you know of a good reference for this?