In one of Thorpe's soon-to-be-legendary breakdowns, Thorpe explains Gilbert Arenas, who he calls the best combination guard in the league, and the third best finisher (Behind Bryant and Wade). Here's Thorpe's explanation of Gilbert's penetrations:
I liken him to boxing legend Joe Frazier -- always coming, always pushing, always in an opponent's face. Arenas puts the defender on his heels. (Try sliding your feet from that position. You can't because you have no balance -- a defender need to be on the balls of his feet to move adeptly.)
Arenas knows how to slow down just enough to keep his defender further off-balance and off-timing, then he habitually makes one of the most fundamental movements any attacking offensive player can make -- he "sits down" before he attacks.
That means he bends his knees a little more and crouches down with his upper body. He gets small, he gets low. Picture a running back exploding through a small hole in the line. Think of a sprinter, starting from a crouch. The lower center of gravity keeps Arenas balanced and also allows him to explode past his defender just a little faster.
So here's the formula that gets Arenas to the basket:
PM + NQ + CS = Blowing past the defender
That's "perfect mechanics" (staying low, with flawless ball handling) plus "natural quickness" plus "changing speeds."
If any of these elements were adjusted, Arenas would be easier to defend.
Other key points: Arenas gets way more points from the line than similar players, and is an excellent three-point shooter.