Bet365 has this. You place a wager, and then monitor your bet. If you bet on a team to win in lets say basketball, and they are up in the second half, you can take the offered cash out amount or let it ride, or wait a bit and cash out more later in the game if your team is still up. This is a good feature if your team is slipping in the end of a game, and you do not want to lose, or you want to take what you can get.