Cut a piece of paper into a long, narrow strip.
Fold in half.
Hold the ends of the paper in your fingertips with the apex of the fold pointing up.
The folded paper should look like a tall, skinny tent.
Now, move your fingers up and down in opposite directions:
Right side down, left side up.
Your finger motion recreates the muscle motion at the ray base, moving the ray back and forth.
Now imagine 20 or 30 rays all in sync like a line of chorus girls... undulating.
And THAT is how many fish propel themselves.