Along the ridge of a crest, an almost level stretch of a mile or more, Jack eased the grip on the reins, and the black responded with a sudden lengthening of stride and lowered his head with ears pressed back flat while he fairly flew over the ground.
Nothing could match that speed.

The strong mare fell to the rear, fighting gamely, but beaten by that effort of the stallion.
Jack swerved in the saddle and looked back, laughing her triumph.
Pierre smiled grimly in response and leaned forward, shifting his weight more over the withers of Mary.

He spoke to her, and one of her pricking ears fell back as if to listen to his voice.

He spoke again and the other ear fell back, her neck straightened, she gave her whole heart to her work.
First she held the stallion even, then she began to gain.

That was the meaning of those round, strong hips, and the breadth of the chest.