Let a connected linear algebraic group $G$ acts on an algebraic variety $X$, proper over a filed $k$.
In the proof of Proposition 1.5. of Mumford's GIT book, he says "....consider the see-saw exact sequence: $0 \rightarrow H^1(\mathcal{O}_{G}^{\times}) \rightarrow H^1(\mathcal{O}_{G\times X}^{\times}) \rightarrow H^0(G, R^1 (p_1)_{\ast} (\mathcal{O}_{G\times X}^{\times}))$". I don't know "the see-saw exact sequence" and cannot find it on the web. What is it? Where are references of it?