Mirko Sandic, the legendary Yugoslavian player, winner of Mexico 1968 Olympic, reveals his secrets during a training session at Barcelona 1970, European Championships. Pictures extracted of the book of Kelvin Juba published on 1972. The picture's description says: "The player in possession is static with his back to the goal, while Sandic is close to him in a high body position. The "chopping action" of the hand to the inside of the attacker's arm is to force him to drop the ball".
Source: Water Polo by Kelvin Juba, 1972