Since the plate is pinned at $\mathrm{C}$, it rotates about the pin.

As the plate begins to rotate, the rod connection at $\mathrm{B}$ moves to the left. Member $\left(1\right)$ is connected to the rigid plate at $\mathrm{B}$; consequently, member $\left(1\right)$ is stretched.

Point $\mathrm{D}$ on the rigid plate has only just come into contact with post $\left(2\right)$. Therefore, the post is not yet strained (i.e., compressed).