A bettor who thinks that the event will occur will buy the bet and a bettor who thinks the event won't occur will sell the bet.
An event could be, for example, the "FTSE to close 11 points higher by midday", "the FTSE will end higher on the day", "Brent Oil will close between 50 and 60 cents lower" and so on.
The FTSE does close higher on the day, meaning that the bet makes up at 100.
The FTSE does not close higher on the day, meaning that the bet makes up at 0.
Therefore the winnings - or losses in this case - are: Example three: The same bet, that "the FTSE will close higher on the day".
The FTSE does not close higher on the day, meaning that the bet makes up at 0.
This allows the taking of an early profit or cutting losses if the bet is going the wrong way.