What it strengthens: 
Abs and back

Cassandra's Note: 
This one feels amazing if you have any back stiffness. I love that it works the back and front of our torsos to encourage balance between the two. This is what I like to call a strengthening stretch since it does both: stretches and strengthens!

Kneeling in a quadrupedal position (wrists under shoulders, knees under hips), breathe in and curl your tailbone up to the ceiling while lowering your spine to the floor and looking up (this is the cow portion). Still support your spine with your abs even though they are extended downwards. When you breathe out, do the opposite and tuck your tailbone under, extend your spine up towards the ceiling, and look down (that's the cat part). I usually describe this as holding a flexible rod by the ends and bending the middle upwards and downwards. Move by following your natural breathing rhythm. Going all the way up and all the down is one repetition, so lather, rinse, and repeat!