What exactly does runtime.Gosched do?

In a version prior to the release of go 1.5 of the Tour of Go website, there's a piece of code that looks like this.

package main

import (
    "fmt"
    "runtime"
)

func say(s string) {
    for i := 0; i < 5; i++ {
        runtime.Gosched()
        fmt.Println(s)
    }
}

func main() {
    go say("world")
    say("hello")
}

The output looks like this:

hello
world
hello
world
hello
world
hello
world
hello

What is bothering me is that when runtime.Gosched() is removed, the program no longer prints "world".

hello
hello
hello
hello
hello

Why is that so? How does runtime.Gosched() affect the execution?