The sema is a structured ceremony rather than a spontaneous dance, built from set stages of greeting, recitation and musical accompaniment, at the centre of which the semazen, the turning dervish, revolves slowly and repeatedly with arms raised, one palm turned upward and one down, a posture the order reads as receiving grace from above and passing it on to the earth. The practitioner wears a tall, conical felt cap, the sikke, said to represent a tombstone, over a long white skirted garment, the tennure, that flares outward as the turning gathers pace; the whole is understood within the order as an enactment of Rumi's teaching that the created world itself turns in remembrance of God, with the human being turning in conscious imitation. This site describes the ceremony as the outsider and the historical record know it, and does not set out how the turning itself is trained or performed, which remains the order's own transmitted instruction.
The ceremony very nearly did not survive into the present.
When Mustafa Kemal Ataturk's republic closed every dervish lodge and banned every Sufi order across Turkey in September 1925, the Mevlevi order lost its legal existence along with all the others, and its practice continued only privately and at real risk for decades. Restrictions eased from the 1950s, and Turkish authorities allowed the sema to resume publicly, but as a presented cultural performance rather than as the freely conducted devotional rite it had been before 1925. UNESCO's 2005 proclamation of the Mevlevi Sema Ceremony as a Masterpiece of the Oral and Intangible Heritage of Humanity confirmed that framing internationally, and secured its survival even though the order's legal status in Turkey as a religious body has never been restored.
Sources The Sufi Orders in IslamJ. Spencer Trimingham with Islamic Mysticism: A Short HistoryAlexander Knysh, Mevlevi Sema Ceremony, UNESCO Representative List of the Intangible Cultural Heritage of HumanityUNESCO Intergovernmental Committee for the Safeguarding of the Intangible Cultural Heritage