Insert the Backer Rod into the Gap

What happens here

Take the selected backer rod and firmly stuff it into the entire length of the large gap behind the benchtop. The presenter uses a utility knife or similar tool to help press the backer rod down securely, ensuring it's positioned to provide solid backing for the subsequent caulk application. This step is critical because the backer rod helps the caulk ('Big Stretch' sealant) perform better by creating a shallow, consistent void for the caulk and preventing it from simply falling through the large opening.

Goal: Create a stable and appropriately sized base for the caulk within the large gap.