They say to smile instead of frown, since smiling takes 15 facial muscles and frowning requires 35. It's a simple matter of efficiency.
Some women take this advice a step further, and laugh instead of push. Every now and then I'll have a woman that is ready to deliver, and instead of pushing, something will make her laugh and she'll just pop that baby out. Instead of screwing up the face and pushing as hard as she can, the woman will give a laugh (creating some abdominal pressure) and the head will come out.
What a fun way to deliver. I love it.