It's fairly straightforward. In addition to the playing XI, a team will have to list four substitutes at the toss. They can use any one of the four subs as their Impact Player.
There are a few options. A captain can nominate the team's Impact Player and bring them in before the start of an innings; at the end of an over; and at the fall of a wicket or when a batter retires. However, if the bowling side brings in an Impact Player during an over - at the fall of a wicket or if a batter retires - then that particular player will not be allowed to bowl the remaining balls of the over.
How much impact will the Impact Player rule have?
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The replaced player will take no further part in the match. Not even as a substitute fielder.
If a team names four overseas players in their starting XI, then they can only bring in an Indian as the Impact Player. This is to limit the number of overseas players per game to four per team - something the IPL has adhered to since its inception. However, if a team starts with three or fewer overseas players in their XI, then they can bring in an overseas player as the Impact Player. But the incoming overseas player should be among the four substitutes named at the toss.
No, it doesn't. Only 11 players can bat. So if the batting team's Impact Player is a batter who replaces a batter who has been dismissed/retired, then one of the players yet to come in - presumably a bowler - will not bat.
When the bowling team brings in their Impact Player, they will be allowed to bowl their full quota of four overs irrespective of the number of overs bowled by the player they are replacing. For example, say a team has a powerplay specialist and bowls them out at the start of the innings. In theory, they could then replace the powerplay specialist with a death-overs specialist - their Impact Player - who can still bowl four overs. But remember, if the bowling team brings in their Impact Player in the middle of an over, they will have to wait till the end of the over before that player can bowl.
The IPL said that there will be no change to the implementation of the Impact Player rule if "a delayed start reduces the total amount of overs available to both sides to less than 20 overs per innings prior to the commencement of the match… Impact Player can be utilised at any time during the match."