Almost every motion you can imagine comes with a counter-motion right before it starts. If you want to stomp on a can of empty soda, you've got to lift your leg up first to really get that stomping impact. Or, if you are standing and want to walk forward, you may lean back just a tiny bit before taking your first step. This is called Anticipation.
Anticipation is really important in animation: it is a contrasting motion to your main motion. Contrast is key, and will help bring attention to whatever you are trying to highlight. (This concept of contrast goes throughout all forms of animation and artmaking). So if you want to make a character really seem like they're running fast, a little counter-intuitive motion of the character leaning back before they start running will actually give a stronger impression of running.
Stay tuned for interactive demo animation!