As you know, HumanoidRootPart is followed rotate with the screen when " shift locked switch"
What line is the rotation value for the HumanoidRootPart in the “camera modules” ?
I tried working with my own local scripts but it was unnatural , so I started looking for the Camera module scripts.
i want to make smoothly rotate body when Shift locked.
like this
The reason you’re struggling to find this value is because it’s not there! 
It’s quite annoying, but roblox actually uses a user settings enum to dictate when the player should rotate in the same direction as the camera and then does the rotation “under the hood”.
The enum is set on line 1125 of the base camera.
UserGameSettings.RotationType = Enum.RotationType.CameraRelative
Good luck! 