Steven drops onto his head and rolls over onto his front, totally thrown by this experience. In reality, though, he's been fighting people all day, and Jasper only just came into her powers... there are a lot of factors that turned this one against him, and if he was thinking rationally he'd know that.

He's not thinking rationally, though. So when Jasper turns to march away, he compulsively turns this into a test of her patience as he throws his shield at the back of her knees.

"No," he still tries to insist. "I... I can win."