Invisible Hand was a lightsaber technique, pioneered by General Grievous and later refined by Tak Sakaros. In involved an initial strike of a lightsaber blade, so blindingly fast as to fatally strike the target before he or she even saw or sensed the attack coming.
Due to the electrodrivers and other mechanical hardware that made up his "body", Grievous was able to execute moves so quick that his striking hand seemed to vanish into thin air, only "reappearing" after the blow had already struck its target. Sakaros, who trained with Dooku and served as one of his acolytes, had the chance to see Grievous in action and observed the General's blows of this nature. Though very few sentient species could ever hope to move as quickly as Grievous, Tak saw the potential for a generalized technique.
Decades later, once he had mastered Makashi in addition to Djem So, Tak made a widely usable technique of it. Using extreme concentration, aggression, telekinesis, and a burst of Force speed, a lightsaber combatant who caught his opponent unwary could draw his lightsaber off his belt with the Force (for to actually pull it off by hand would take too long), activate and catch it in midair, and already be moving to strike his opponent with the newly activated blade. Sakaros himself preferred the "Invisible Hand" strike to be a lunge, as a pinpoint strike (as opposed to a broad slash) made the chances of an enemy deflecting the attack even less likely than they already were.
Aside from Sakaros himself, his sons Khrado and Tariun and later his daughter Rin were known to have mastered the technique.
Behind the scenes[]
The electrodrivers that powered Grievous's limbs could move them faster than the human eye can see; when he swung his arm, it and his fist and the lightsaber would literally vanish: wiped from existence by sheer mind-numbing speed, an imitation quantum event.
—Matthew Stover
The above quote from Matthew Stover's novelization of Revenge of the Sith is the basis for "Invisible Hand". The name itself is taken from Grievous's flagship, as well as a further quote from Stover's novel:
...and as one of the MagnaGuards' electrostaffs fell past [Grievous], his invisibly fast hand snatched it from the air.
—Ibid