"Just like in checkers, jumping opponent's pieces is mandatory. It means that if a player's piece stands next to an opponent's piece and can jump it, it is forced to do it and it must continue jumping as long as it can."
It is near the bottom of the page, under "Other important rules"
(hide) Play a game in real time with an online opponent! For this to be possible, you and your opponent have to select the action "Move and stay here" as default and then reload the page with the F5 key! (TeamBundy) (show all tips)