In the beginning of this scene, a group of people bursted into the dorms and told everyone to get up and meet them at the train tracks
When they get on the train, Eric and Four divide everyone up into two groups for capture the flag, a Dauntless tradition
Four and Eric's team split up, and Four's team ended up in an abandoned park, looking for somewhere to hide their flag
While everyone is aruging about whether to split up, Tris decides to take things into her own hands and decides to climb the old ferris wheel to get a good view of the other teams side. Four follows, even though he is afraid of heights.
While they are climbing the ferris wheel, they spot where they other team hid their flag. As they are climbing down, Tris slips on a bar,and it breaks underneath her. She almost slips and falls, but Four luckily starts the ferris wheel, and gets her down safely.
Tris and Christina went to go get the flag, but Tris felt it was only fair if Christina takes the credit.