Here is how they are supposed to work, based on a supposition that the frame speed is 24 frames per second.
You create your starting pose, A...
... and your ending pose, B.
Then you do this weird inbetween drawing that will appear on one frame only; this is very important.
If the movement is left to right, you trace the left contour of drawing A, then the right contour of drawing B.
|Contour A shown in Red.|
|Contour B shown in Blue.|
Between those contours you handle the shape like a piece of taffy stretched across between the starting and ending contours.
As appropriate, include an arc of movement in this drawing.
The result will be a smear or blur that can be a quite effective transition. The viewer will not be able to focus on the inbetween but the effect will be of a smooth, although lightning fast, movement, rather like that of a bird suddenly moving its head.
|Here is the smear tween laid over the two key drawings.|
|This is the smear tween alone.|
|Above, the three images in color.|
Let's now look at a video of this effect, created in Flipbook through Autodesk Sketchbook.
Note: For the best effect, try looping this video. See instructions at the top right of this page if you don't already know how.