「動けば雷電の如く、 発すれば風雨の如し」is not a quote from Takasugi Shinsaku. It’s a phrase dedicated by Takasugi’s coleague in Yoshida Shoin's private school, 松下村塾, and then Japan’s first Prime Minister, Ito Hirobumi as a part of the epitaph engraved in the monument of Takasugi Shinsaku situated in a corner of 東行庵 in Shimonoseki, Yamaguchi Pref. It was built on May 20, 1911, 44 years after Takasugi's death and two years after Ito's death. You can visit and look at the epitaph there today.
The paragraph of the epitaph in question reads;
which can be translated as;
“When Takasugi moves, it’s swift like a thunder and lightning. When he utters, it’s fierce like a storm and downpour. Everybody was overwhelmed, and nobody ventures to look at him straight in face. Togyo (a nickname of Shinsaku) Takasugi was a man exactly like this.”
though I'm not very sure of whether 発する
can be translated as 'utter a word’.
The original characters of epitaph was written by Sugi Magoshichiro, and engraved by a mason.
The author, Ito Hiobumi was assasinated by a Korean activist in front of Herbin Sation in Manchuria on October 26, 1909 without seeing the completion of the Takasugi's monument on which the epitaph he drafted was engraved.