Francisco Rodriguez currently has 31 saves in the 81 games the Angels have played. That means he’s on pace for 62 saves this year, which would break Bobby Thigpen’s single season saves record of 57. Could K-Rod break the record? Absolutely, but I’d bank on him not making it that far. So by that logic, you’ve already banked his best half and now is the time to trade him away. He’s been so good and his numbers are amazing, so you should have no trouble at all getting solid return for him. Not to mention you’ve already banked those 31 saves and should have some wiggle room in the saves department because of that.
Nice example. I believe it's called the "take your winnings and run" principle.
Also see Contreras, Jose and Ludwick, Ryan.