I seriously just watched that GIF repeat for a solid 5 minutes.
I have a folder marked swings to study. I'm going to put this one in that folder. I usually open the clips in quicktime and loop the clips, then check the frame by frame.
One thing is he shifts the weight back into the hip joint with stabilization happening at the same time. The girl shifts back until the joints are stacked and then pauses. She is shifting and swinging at the same time. He is FBC with the joint always out front of the back foot. He is always working forward even though he is also getting the weight back, coiling.