I'm running on 2.8 and I'm not sure how you get the grease pencil to change depending on what frame I'm on as you sketch out the jump animation, or is this only possible in the reference video?
HI Nomad.
This will work in the clip editor (on the reference video) or in the 3d viewport as well.
If you use the annotation tool you have less controls over things, but it will automatically show the current drawing until you get to the frame where the next drawing was scribbled in.
If you actually use grease pencil, make sure you draw inside the grease pencil object in Draw mode. And it works the same way, but you have more functionality.
If you want to get your video ref into the 3d view, you have 2 options. Use the "Import Images as Planes" add-on (ships with Blender you just need to enable it).
Or you can attach it as a background image/video to a camera.
I think just drawing in the clip editor is the easiest way to go.