I'm looking at the original Foldy Wouthuysen article found here http://www.physics.drexel.edu/~bob/Quantum_Papers/Foldy-Wouthuysen.pdf, and have some question regarding eq. 31 in this paper. Would anyone be able to explain how this is derived? In particular the part with the time derivative of the exp(-iS) term. The wiki-article on FW transformation is rank with typos and also glosses over this point.