International Institute of Cricket Umpiring & Scoring
Distracting the strikerAny attempt by any fielder deliberately to distract the striker or obstruct either batsmen is unfair play.
Distraction or obstruction can be by word or action, or both. If a fielder wilfully attempts to distract the striker - and it doesn't have to be successful - while the bowler is running in to bowl or the striker is in the act of receiving the delivery, either umpire should call and signal Dead ball, issue a first and final warning to the fielding captain and tell the other umpire and the batsmen what has happened. The ball will not count as one of the over.
If it happens again the umpire should repeat that sequence of events, additionally awarding 5 Penalty runsto the batting side and reporting the occurrence to the appropriate authorities as soon as practicable.
If the distraction or obstruction occurs after the striker has received the ball, a similar sequence of events takes place, except that there is no warning this time and neither batsman can be dismissed from that ball, which will not count as one of the over, the 5 Penalty runs are awarded immediately, and any run in progress will count even if the batsmen had not crossed. What's more the batsmen at the wicket can decide which of them is to face the next delivery.
Close fielders even have to be careful of their shadows moving across the pitch and distracting the striker and, if there is the possibility of this happening, they should remain stationary until the striker has played the ball. If the striker seems to be being distracted, Dead ball should be called.
All of this adds up to several excellent reasons why distracting or obstructing batsmen is not a good idea!
Read more about Law 42.4 (Fair and unfair play) at the MCC website