Very cool, but I'm not sure where the article got the 8,800mph number. From what I understand the Earth relative speed is around 23 or 24 thousand mph.
Juno launched by Atlas 551 rocket, but that powered only as far as the asteroid belt. Then the Sun sucked Juno back in, effectively causing Juno spacecraft to orbit the Sun. The Earth flyby was planned as a trajectory to increase Juno's speed relative to the Sun so it can reach Jupiter. Earth's gravity causes a change in velocity to the spacecraft relative to the Sun, as much as a second Atlas 551 would give. Like a ricochet/slingshot.