In the video above, you can see a white contrail entering the left of the frame. That's not just the HTV-2, but the third stage of the Minotaur 4 rocket that carried HTV-2 to the edge of space. From there, if you look very closely you can see HTV-2 separate from the rocket stage (it's a really faint dot) and begin its aerodynamically stable hypersonic flight, in which it hits its objective speed of Mach 20.