This is a timelapse video of me creating and "inbetween" animation drawing between two "key poses" of animation in about 10 minutes.

I flip constantly to check and see that the lines are moving correctly.

At about 1:30, you'll see me shift the drawing then do a quick sketch then return it back to its original position this is a method called "shifting & tracing". We use it to reference the shapes from the other drawings in order to help us maintain the volume of mass of the character from drawing to drawing so that the animation movement feels solid and stable.