[Peruvian]

(11) Runner on third base, one out. Batter hits a fly ball to right-center field, and the runner goes back to third base to tag up. Third baseman approaches the runner, faces him, and jockeys back and forth, intentionally trying to block the runner's view of the fielder catching the ball.
Ruling: This is obstruction under Official Baseball Rule 7.06(b). The umpire should call the infraction when it occurs and award bases, if any, in the appropriate manner.

Well, that just about clears it up for me. Wish I had a copy of that book.