Again...from Yahoo help:
However, you will only be able to add players to your active roster for the current week if the transaction is completed no later than five minutes before the start of the player's game. In other words, none of the active players on your team (players in a starting roster spot) involved in a move can have played or be within five minutes of their game's start time, or the transaction will not be allowed. So, for example, transactions for all active players who play at 10:00 am PT must be completed by 9:55 am PT.
"Give a man a fish and he will eat for a day; teach a man to fish and he will sit in a boat and drink beer all day" - Confucius' redneck brother