With menstrual cups, a seal is created where the vaginal walls meet the cup wall. Once this seal is created, if you were to tug on the cup, you would experience suction.
To remove a traditional cup, you have to reach inside to break this suction. With the FLEX Cup, all you have to do is pull the ReleaseRing™ which breaks the suction, and the FLEX Cup slides out easily.
For more information, check out this "How to" video here. Just be sure to click "FLEX Cup" under Choose a product.