In-play betting lets a bettor place bets while a match or race is under way. Prices move with the score, the time left and key events, and markets can be suspended for a few seconds after a goal or a point. The speed and the constant chance to bet make it easy to wager more than planned. Setting a limit before starting is a useful safeguard. Fast changing prices also mean the margin built into them can be larger than in pre-match markets.
Example
A team priced at 2.50 before kickoff may move to 1.30 after scoring early. A new bet at 1.30 has an implied probability of about 77%.