My biased theory is that Kimimaro aims his ultimate attack at Rock Lee knowing that he can't defeat Gaara one on one. Gaara tries to make up for the past by taking up the hit for Lee.

Now Rock Lee goes berserk mode, similar to when Naruto turned into Nine Tails after Sasuke took the hit, but much more powerful of course.

Lee attains light-speed taijutsu that would rip your skin apart just by running pass by you, and shows Gaara the true meaning of friendship by owning the sh!t out of Kimimaro times 299,792,458 meters per sec.