When to proof P→Q true, we proof P false, that type of proof is known as ___________

This type of proof is known as "Proof by Contrapositive." In this method, to show the implication $P \rightarrow Q$ is true, one often demonstrates that if $P$ is false, then $Q$ must also be false. This is based on the logical equivalence that $P \rightarrow Q$ is equivalent to $\neg Q \rightarrow \neg P$. Thus, if we can establish that $P$ being false leads to $Q$ also being false, we confirm that the original implication holds true.

In summary, the process relies on the logical framework that indicates the truth of implications can be effectively explored through their contrapositive relationships.