Answer: "Run, Forrest, run!"
At first, the three bullies just taunt Forrest and he slowly walks away, encumbered by his leg braces. But after the bullies get on their bicycles, Forrest must move away more quickly to escape them. Somehow, he doesn't think of running until Jenny tells him to (or doesn't think he can, but Jenny is always his inspiration). When she does, he can then run so swiftly that the braces on his legs 'magically' fall off and he runs ten times faster than any bicycle!
"Run, Forrest, run" is probably one of the most quoted lines from the film. Jenny also gives Forrest similar advice later in the film, when she learns Forrest is going to Viet Nam. He heeds that advice and would not have been shot but for the fact that he cannot leave his buddy, Bubba, and goes back to find him.