Windu had disarmed Sidious and cornered the treacherous Sith Lord into the frame of his expansive office window. Fearing that Sidious was too powerful to take prisoner -- the Dark Lord still had control of the Senate and courts -- Windu moved to deliver a deathblow. Torn between his loyalties, and needing Palpatine alive to fulfill his quest for arcane knowledge to preserve the life of his beloved, Anakin acted. In an emotionally charged action, Anakin ignited his lightsaber blade and sheared off Mace Windu's weapon hand before he could strike Palpatine.
Defenseless, Windu was bombarded with Sith lightning as Sidious unleashed a torrent of deadly energy at the Jedi Master. The forked bolts of lightning penetrated Mace's body, illuminating his form from within. The final blast bodily lifted him into the air, sending his form hurling into the Coruscant skies, to crash lifelessly somewhere in the vast cityscape below.
Everyone seems to interpret these differently.
Seems to me that Mace was the victor of thier duel, and only Anakin's interference gave Sidious the upper hand. Only when he was "defenseless", or without his lightsaber was he prone to the lightning. Maybe only Yoda, and I guess Anakin at some point, would be strong enough to stop the lightning without sabers.
The Jedi eventually discovered Palpatine's Sith leanings and confronted the Chancellor. Sidious lashed out with blinding speed, brandishing a previously hidden lightsaber blade in a sudden strike that killed all of the Jedi Masters Mace Windu had assembled to arrest the Chancellor. With Anakin Skywalker's help, Sidious was able to defeat Mace, though he was severely scarred by the reflected power of his dark side lightning. To conceal his disfigured visage, Sidious returned to his simple Sith robes.
Here it says that Sisious needed Anakin's help to beat Mace... this one is in Sidious's date base entry, but again we are all going to take what we want from these.