Salem, you never detach an archer from their bow. Katniss's grip tightens, which is why she isn't focusing on where his feet are. She can feel her knee start to give up, and start to fall, so she keeps her grip on the bow as she stumbles. She hits her knee, hard, which is fine, because now she uses the bow to shove up under Salem's chin hard enough to jar him back. Using her height and momentum to hopefully get him back away from her.