Most swimmers begin their butterfly pull by moving their hands sideways and away from the body. The university male swimmer in the video image below is a typical example. Note how the Aquanex analysis shows that after one-quarter of a second, he is still only generating about 4 lbs of hand force.

Butterfly Examples Aquanex

In contrast, the high school female swimmer in the image below flexes her elbows to begin her pull. Because her arms are immediately in a stronger position, she is generating three times as much force as the much stronger, male swimmer above.