The trivia item below may give away important plot points.
The spike and chain is a method used to condition young elephants. Shackled to a short chain with the spike driven into the ground, the young elephant struggles until it breaks the skin around its shackled leg. That painful memory is used to control the full grown elephant, even though it could easily pull out the spike as an adult. Rosie accepts Jacob by giving him the spike and ultimately kills August with the same.