Murphy's Law in action: as soon as I promised to make a video about this, my dog hurt his foot. It ended up not being a huge deal since I wanted to do a super slowed-down version anyhow, but ordinarily Pongu does a literal flip (jump) into position and he is not doing it in these youtubes because he chewed a hole in his foot so I'm not encouraging him to jump.
Step 1, as a couple of people mentioned up-thread, is to be sure that your dog has a very clear understanding of Heel position.
One way you can test this is to run a diagnostic exercise: close your eyes and hold yourself in the default position that you would have while your dog is in Heel (which for me is standing with shoulders straight, looking ahead, left hand over midsection -- for you it might be a little different, depending on your sport and region). Give no cues to your dog besides your posture and a call to Heel (this is why you have your eyes closed, so you can't cheat). When you open your eyes, where is your dog?
If the answer is "in Heel position," the next step is to test whether your dog can find that position from a variety of directions and with you standing in different orientations. To get more repetitions in a little faster, you can throw the treat to draw the dog off instead of rewarding in position. (NB, though, that this can erode the dog's position over time -- it's always a tradeoff between rewarding for position and rewarding to obtain movement.)
Here's Pongu doing the diagnostic exercise:
If your dog can't find Heel position reliably, or you're not totally happy with some aspect of his Sits (such as: I taught Pongu a rock-back Sit as a puppy and I'm still
suffering for it), then go back and work on that first.
Step 2 is the actual teaching of the left/flip finish. I did this by getting Pongu to follow a hand target.
I find it easiest to draw the dog slightly outward when the dog is moving away from you, then back in when the dog is moving back toward you (so the dog is turning inward to reach the final position). This brings the dog in closer; if you do it the other way (turning the dog outward) you'll probably end up with a wide Sit.
If you want to encourage the dog to leap flashily into position, one way to do that is to raise your hand targets over the dog's head so that the dog is leaping up to hit it, and reward the repetitions where the dog is jumping instead of walking into position. As with all jump moves, I wouldn't recommend doing this with a puppy, or with any dog repeatedly on a hard surface.