A more thorough explanation:

PROPRIO VIGORE

Proprio vigore (proh-pree-oh vi-gor-ee) is a Latin term that means "by its own strength."

1. The contract will terminate proprio vigore on the expiration date.

2. The law will come into effect proprio vigore on January 1st.

The term proprio vigore is often used in legal contexts to indicate that something will happen automatically or without the need for external intervention. In the examples given, the termination of the contract and the coming into effect of the law will happen on their own, without any further action required. The term emphasizes the self-sufficiency and self-determination of the thing in question.