Ponyboy is still wavering somewhat as the rumble is about to begin. As Darry and Paul "moved in a circle under the light," Pony watches them and tells himself
"I don't hate the Socs any more..." (Chapter 9)
Just then, Dally makes his entrance. Hearing Dally's "familiar voice," Darry "turned" and took
... a hard right to the jaw that would have felled anyone but Darry. (Chapter 9)
So, it costs Darry a solid punch to the face, but Dally soon proves his worth. The three combined greaser gangs are still outnumbered by the four carloads of Socs--twenty-two strong. Dally gives the greasers a twenty-first man, making the sides nearly equal. Dally's appearance clears Pony's head, and he quickly finds a Soc
... and jumped on him. Dallas was right beside me... (Chapter 9)
The two boys double-team one Soc, and when another Soc gets the better of Pony, "Dally knocked him backward." Pony takes quite a beating, being kicked so hard in the head that he "lay limp." When he looks up at the Soc who is kicking him, Pony sees "Dally was slugging him." Dally's late appearance gives the greasers an extra man, fires the boys up, and provides Ponyboy with an experienced rumbler to watch his back.
It gives the Greaser more motivation and satisfaction. At that point the Greasers already thought of winning. The Socs probably got scared since they didn't expect him due to the fact that he was in the hospital [newspaper].