No need, here's the exact passage:
So now, facing the tornado of annihilating energy that is Grievous's attack, Obi-Wan simply is who he is.
The electrodrivers powering Grievous's mechanical arms let each of the four attack thrice in a single second; integrated by combat algorithms in the bio-droid's electronic network of peripheral processors, each of the twelve strikes per second came from a different angle with different speed and intensity, an unpredictably broken rhythm of slashes, chops, and stabs of which every single one could take Obi-Wan's life. Not one touched him.
After all, he had often walked unscathed through hornet-swarms of blasterfire, defended only by the Force's direction of his blade; countering twelve blows per second was only difficult, not impossible. His blade wove an intricate web of angles and curves, never truly fast but always just fast enough, each motion of his lightsaber subtly interfering with three or four or eight of the general's strikes, the rest sizzling past him, his precise, minimal shifts of weight and stance slipping them by centimeters.
Grievous, snarling fury, ramped up the intensity and velocity of his attacks-sixteen per second, eighteen-until finally, at twenty strikes per second, he overloaded Obi-Wan's defense. So Obi-Wan used his defense to attack.
What's interesting is that when Grievous' APS did reach 20, Obi-Wan's defence was described as "overloaded," and he was forced to go on the attack.