In this recipe, you added a FreeLook Cinemachine GameObject, but with a priority of zero. So, initially, it will be ignored. When the 1 key is pressed, the script increases the Priority to 99 (much higher than the default of 10 for the ClearShot cameras), so then the Cinemachine Brain will make the FreeLook virtual camera control the Main Camera. Pressing the 2 key reduces the FreeLook component's Priority back to 0. So, the ClearShot cameras will be used again.
There should be a smooth transition from FreeLook to ClearShot and back again since you set the Default Blend property of the Cinemachine Brain component in the Main Camera to Ease In Out.