How to: seamless transition?

Good camera work and experience.

It's just an animated mask in After Effects. Essentially the first clip is above the second clip in a timeline, and when the camera passes the guy's leg, they animated a mask so that anything to the right of the leg line (where the mask was drawn) is removed from the image, revealing the second shot below.

Same concept as this, just more complex.

https://www.newschoolers.com/videos/watch/793205/Animate-Masks-in-Adobe-After-Effects-Tutorial-by-klipovski

I hope you have/know how to use AfterEffects. Its quite simple if you know how to do it. Place the pan clip over the rider clip. When the camera pans past the guys legs, roto or pen tool mask the edge of his right leg, so the rider clip can be seen on the right side of his leg, perfectly spiced in. Feather the mask probably 2 pixels, then animate the mask with keyframes to move with the standing guys leg and boom sick shot.