"I choose to be strong by being soft and pliable rather than inflexible, brittle, and hard."
A man is born gentle and weak; at his death he is hard and stiff. All things, including the grass and trees are soft and pliable in life; dry and brittle in death.
Stiffness is thus a companion of death; flexibility a companion of life. An army that cannot yield will be defeated. A tree that cannot bend will crack in the wind. The hard and stiff will be broken; the soft and supple will prevail.