What's this?

[John Robertson]

I'm assuming when you wrote "backswing" you meant the action of a player who has already swung at a pitch and is now bringing the bat back.

If this is the case, I can't call it a foul ball. It's a swinging strike followed by a bit of unintentional interference by the batter. If it's strike three, the batter is out and the ball is dead. If runners are attempting to steal on the play, you might have a good case for calling interference too.