Using legs

Whenever both of your arms point up, your legs automatically start kicking, giving you some easy speed in confined spaces. If your arms aren't up, you have to pump them manually.

Using arms

As mentioned, having your arms up is useful, but if you pump your arms as if you're swimming, you'll get more of a speed boost than if you were just kicking. Add some manual kicking to that, and now your a speedy boy.