Darvesh is derived from the word Dar which means 'the door' in Persian, hence a Darvesh is the one 'who opens the door'.
Rumi has said in reference to Sema, "For them it is the Sema of this world and the other. Even more for the circle of dancers within the Sema Who turn and have in their midst, their own Ka'aba." and what he is saying is that when, like in Mecca you have come closer to God, likewise when you perform Sema you are also closer to God.
The Sema was practised in the semahane (ritual hall) according to a precisely prescribed symbolic ritual with the dervishes whirling in a circle around their sheikh, who is the only one circling around his axis. The Sema is performed by spinning on the right foot.  The dervishes wear a white gown (symbol of death), a wide black cloak (hirka) (symbol of the grave) and a high brown cap (kûlah), symbol of the tombstone.
Sema ceremonies are broken up into four parts which all have their own important meanings.
Naat and Taksim - Naat is the beginning of the ceremony where they sing praise for the Prophet Mohammed. The first part is finished with Taksim or the playing of the reed flute which symbolizes our separation from God.
Devr-i Veled - Devr-i Veled is when the dervishes bow to each other. The bow is said to represent the acknowledgement of the Divine breath which has been breathed into all of us. After all the dervishes have done this they kneel and recite a short dhikr before removing their black cloaks.
The Four Selams - The Four Selams are the central part of Sema. The semazen, or the whirling dirvishes, are representative of the moon and they spin on the outside of the Sheikh who is representative of the sun. They, as previously mentioned, spin on their right foot and additionally, they have their right palm facing upwards towards Heaven and their left hand pointing at the ground. The four selams themselves are representative of the spiritual journey that every believer goes through. The first one is representative of recognition of God, the second one is recognition of the existence in his unity, the third one represents the ecstacy one experiences with total surrender, and the fourth one, where the Sheikh joins in the dance, is symbolic of peace of the heart due to Divine unity. After the four selams, this part of the ceremony is concluded with another Taksim.
Concluding Prayer - The fourth part of the ceremony is a recitation from the Holy Qu'ran and a prayer by the Sheikh and then the Sema is complete.
The only live performance I've seen was last year at the Sarkhej Roza. It was beautiful, but I saw it just like a performance. Wonder if I would ever be able to actually experience the trance...