He doesn't use force lightning. What was happening was that Sidious was using force-lightning on Yoda, and Yoda was absorbing it/storing it in his hands. Now, when Yoda looked away, Sidious thought he was faltering because he couldn't absorb any more, but Yoda turned back again to show Sidious that he was still strong. It wasn't a dark-side technique, Yoda was just displaying that he was capable of absorbing all of that power. And then, Yoda, who had absorbed all of the force-lightning, reflected it back at Sidious, which caused that strange explosion. This is similar to how Yoda reflected the lightning that Dooku sent at him in Episode 2.
Or at least that's my understanding.