You get the purchased miles at purchase and the flown miles whenever they decide to post them after flying.
That is how they arrive at what appears to be "single miles" for purchase being double miles, and what appears to be double miles being triple miles. You buy 2x extra miles, fly and earn 1x miles, for a total of 3x miles. Now if you cancel the ticket and don't fly it, you still keep the Accelerator miles you paid dearly for, but don't earn the flown miles of course.