Just to clarify on this, the reader can go back to the video from up above with the guy crimping the glass vial with stopper and west pharma seal.
If the vial is properly crimped then the aluminum shell is providing a significant normal force
pressing the rubber stopper flange against the glass vial lip. The coefficient of static friction (and resultant fractional force due to applied normal force) between rubber and glass provides the resistance to any applied torque on the aluminum shell as one tries to rotate it.
So if the aluminum shell is freely rotatable, that means that either (1) the stopper is also rotating with respect to the glass vial or (2) the aluminum shell isnt even really engaging the rubber stopper and the aluminum is slipping over the rubber stopper. In either case, the applied normal force by the undercrimp is inadequate for the job.
Fwiw.
View: https://youtu.be/7NyVKMOSuOM