Squash and stretch.

Squash and stretch is the principle that during movement the physical shape of something changes through either stretching or squashing, however the mass stays the same. This seems simple but can easily be done wrong, either over stretching, over squashing or other other way around.

A good example of this is can be seen in the flash game Robot Unicorn Attack.

Comparing the Robot Unicorn from the two images, you can see how the body section of the character stretches and contracts as a horse's does, as well as the legs stretching around showing somewhat realistic movement.

An example of poor squash and stretch can be seen in Fifa 13.

Football's are soft compared to rocks or metal. However, in this footage from Fifa 13, the ball squashes and stretches about as much as an iron girder would if you kicked it. In EAs defense, the majority of the game is at far distance, so squash and stretch on the football isn't such a priority.