(Ooc: Being as your timeline conflicts with either prepared ones, aka you're going airborne, so I will now react to this established timeline of you going first airborne- Rather than stumbling or immediately falling forward)
Finn's body would have reacted to either expected outcomes. What he couldn't have known, though, was that Hikaru would go airborne and away from him. As Hikaru's feet left the ground, his body would lightly crumple from the blow. He had indeed taken the attack head on- However, he had not immediately dropped to his hands and knees, nor had he stumbled back, as was expected. He literally went airborne. This eliminated the immediate idea for either of Finn's plans- But it was still something that he could react for.
Finn's foot had landed back on the ground after the spinning back kick. Immediately, he'd push forward and dash after Hikaru. The moment that Hikaru landed on his hands and knees, Finn was already lifting his leg up- Having stopped before him at the perfect distance for this kick. Hikaru looked like wanted to push off and run- However, the man had absolutely no (or bare minimum) oxygen coming into him. His abdominal muscles and pectorals would be straining, his muscles begging for oxygen. He would not be able to simply stand and run away- And certainly not as quick as he tried. His first instinct would be to fight for oxygen, unable to escape.
Finn's foot was already brought down. The rise and fall of an ax-kick was a single motion. The foot came up in an arc to avoid the object (or in this case, person) in front of them, and then drops back down in a single motion. It offered minimum delay- And due to that, a man with no air in his lungs would not be able to dash away from it. This would result in the ax-kick landing on the back of Hikaru's head, snapping his neck down and knocking the consciousness from him with a vital blow.