He's never seen Spike like this before, and it's quite a revelation. For the first time, he's witnessing the raw power and speed of a vampire at his peak, and it's thrilling. A part of him wants to just capitulate, let that power take him where it will.
The other half of him is getting punched in the face and taunted, and there's no way he's taking that lying down (pun intended). He shakes out the blow and lets the sword clatter to the ground (swords have never really been his thing anyway). "Oh, I'm not done yet!" he assures Spike, and aims a sharp retaliatory punch up at Spike's jaw try and get his weight up off him and roll out from under.