Page "Sacrifice fly" Paragraph 9

During a sacrifice fly, a runner can attempt to advance bases right as a fielder touches the ball.

If the fielder bobbles the ball, the runner may still tag up, even before the fielder has full control of the ball ( before the fielder is credited with a putout ).

The rule works this way because a smart fielder can theoretically purposely bobble the ball and run with it at the same time towards the infield and only completely catch it when he is within a good throwing distance to home plate.

As addressed in the Official Baseball Rules, this was put into the baseball rulebook so it prevented these fielders from intentionally bobbling the ball.