The International Football Association Board, the organization responsible for setting the rules ahead of the World Cup, announced that "feinting" right before kicking the ball during a penalty is "unsportsmen like behavior" and worthy of a yellow card.
Jerome Valcke, a spokesperson for Fifa, said during a recent press conference on the rules set for the World Cup, "Feinting in the run-up to take a penalty kick to confuse opponents is permitted... However, feinting to kick the ball once the player has completed his run-up is now considered an infringement." What this essentially means is if a penalty-taker, for example Cristiano Ronaldo (a common user of the "feint"), scores on a penalty when using a "deke" after the run up but before kicking the ball, the penalty will be retaken and the penalty-taker will then be cautioned.
Players apply the "deke" or "feint" at the end of the run up during a penalty to try and fool the goal-keeper to reveal the direction of their dive, allowing the penalty-taker to calmly slot the ball into the opposite side of the goal.