[In Sebastian's case it's not getting the wind knocked out of him (the wind isn't knocked out of him) that has him remaining on the ground for those first few seconds. It's more the instinct to protect Ciel, to make sure nothing will suddenly snatch him away after that sudden unexpected projectile adventure.
They have the same idea, though - to disentangle to make sure the other is okay. Sebastian just has the idea a moment after Ciel does, so-- uff. That's an elbow in his solar plexus. Sebastian obediently lets go. Go on, use him as a kneading pad all you like.]
What's going on? What happened? [What's with the coming flying straight at him?
For someone who was supposedly just impaled, Sebastian is remarkably non-bloody and in one piece.]