Powerplay in one day cricket has been confusing fans around the world ever since it was first introduced in 2005 and on 1 October 2011 onwards, few clauses were added. From December 2012, ICC has re-revised the rule once again. here's how the new power play rule stands..
An ODI has two innings, of 50 over each, in which first 10 overs are mandatory field restricted which means no more than 2 fielders can be outside the 30 yard circle and also, two fielders HAVE to be at catching positions.
Previously, two blocks of 5 overs, known as batting powerplay and bowling powerplay, opted by batting and fielding side respectively, was the rule but now, no bowling powerplay is required, just the batting powerplay which should be completed before 40th over. Once called for, no more than 3 fielders will be outside 30 yard circle but this time, no catching fielder is required although fielding captain may well choose to.
Note that if there's no power play in progress, maximum of 4 fielders can patrol the boundary in one day internationals.
More over, each bowler now has two bouncers each over. Now, what was happening in first 7 years since this rule first came into picture is that captains were not experimenting enough. Fielding sides were calling their bowling powerplays straight after first 10 overs to limit the damage in case batting side if off to flying start and batting side often called their batting powerplay as late as possible so that they can collect more runs freely.
October 2011 onwards, ICC has twicked the rule a bit by forcing captains to call batting and bowling powerplays inside 16th to 40th over. This means once first 10 overs of mandatory field restriction is over, bowling side can't call for PP. If they want to, they'll have to wait till completion of 15th over. On other hand, batting side will have to call batting PP on or before 36th over. This means overs 11th to 15 and 40th to 50th cannot be under powerplays!
Note that this rule applys only if the innings is off full 50 overs. In case of rain or other factors for which match has been reduced below 40 overs an innings, new changes won't be applied.
In case a match has been shorterned, following is how the powerplay overs are revised.