In the "Butterflies" category, I think it was referring to the sport of swimming in the butterfly style. The acceleration of the swimmers is due to how their position is made due to their movement. The bigger their strokes, the faster they move. This is caused because of the force the swimmer gives when s/he delivers power at three times, twice with their feet and once with his/her hands. Each time the swimmer pushes force against the water, s/he accelerates but as s/he does this, the drag from the water also slows him/her down. Something I thought was interesting in "butterflies" is that in order to travel 2 meters a little of a second you would have to have to use your arms and legs like an expert but your streamline has to be up to Olympic standards.
