It's understandable that you might feel awkward asking your massage therapist about their specific actions during a massage. However, communication with your massage therapist is essential to ensure that you feel comfortable and that the massage meets your needs.
The reason a massage therapist may flip over the liner at the top of your underwear is to ensure that they can properly access and treat the muscles of your lower back and glutes during the massage. By flipping over the liner, the therapist can avoid any obstruction that the underwear's elastic band might create, allowing them to provide more effective massage techniques in those areas.
It's important to remember that professional massage therapists are trained to prioritize your comfort, privacy, and modesty throughout the session. They should always use draping techniques and only expose the specific area they are working on, while keeping the rest of your body covered. If you feel uncomfortable with any aspect of the massage or draping, do not hesitate to communicate your concerns to the therapist.
Open communication is vital in any therapeutic relationship, including massage therapy. If you have questions or are uncertain about any aspect of the massage, feel free to ask the therapist before or during the session. They will likely appreciate your feedback and be more than willing to address any concerns you have to ensure you have a positive and comfortable experience.