OK, kickback clause isn't the right expression.
What I was getting at is that when a player under the age of 24 has been offered a new contract by his club and they reject the offer to take up the opportunity to sign for another club, compensation is payable.
Quick search brought up a couple of examples.
- Anthony Straker joined Aldershot aged 18 having been released by Crystal Palace and went on to make in the region of 200 appearances in the Conference and League Two. He rejected a new contract, and aged 22 signed for Southend United on a two year deal.
Southend will also pay Aldershot 20% of any profit it makes from selling the player to another club at any point in the future.
- Robert Hall had signed a professional contract at West Ham aged 17, and made four first team league appearances for West Ham. He also enjoyed successful loan spells, including in particular at Birmingham City and represented England at under 16 through to under 19 level. Allowing his contract to expire, aged 20 he signed a three year contract at Bolton Wanderers.
20% of any profit made by Bolton Wanderers in selling the player to another club at any point in the future will be payable to West Ham.
- Aaron Creswell had made 70 or so league appearances for Tranmere Rovers when, aged 21, he rejected a new long term contract offer. He signed for the Championship club Ipswich Town on a three year contract
Tranmere Rovers are to receive 20% of any profit made by Ipswich Town in selling the player to another club at any point in the future.