Take another look at the sentence:
Swinging at the tennis ball, contact with the racket sent a stab of pain
through Paul's elbow.
Swinging at the tennis ball, a participle phrase, comes right before contact, an illogical target. Paul is the only one in the sentence who has arms to swing with! Before you continue, review the rules for misplaced and dangling modifiers.
Go back to the sentence to try again.