Akira was taken by surprise at Yui's sudden determined attack. Instinctively he put his arms up to block her feint, not expecting the kick to his groin.

So, that happened.

He hissed in pain and stumbled backward, doubling over. The moment of pain left him wide open for another attack. Looking back on this he'd be grateful for that -- if he were capable of retaliating, he'd pretty much have to. Otherwise there would surely be hell to pay.