The Role of Drums in Spirit Possession;

The concepts and ritualistic behaviors surrounding Vodou drums are related to their capacity as agents of spirit possession, although this process can occur without drums. 

The most thorough and integral explanations of the relationship between drums and possession are given by Vodou servants themselves. The Lwa (Spirits) love music, and each has their favorite beat. From their locations in the Cosmos, spirits respond to a complex of invitational signals sent out by the servants, but the drum might be the most compelling of these. Driven to dance, spirits use their powers to displace, and replace, the souls of dancing Hounsi's (Vodoun serviteurs not yet initiated). 

When a Lwa is present in ceremony, the drummers must keep going, or risk the irritation (ire) of the Lwa who came to dance. Collective energy is a necessary condition for summoning the Lwa, and the drummers use a driving beat to generate it. More specifically, they use a pattern called "kase" (to break) literally to pound a spirit into a servant who shows signs of the onset of possession.

The drummer can permit the tension to build to just the level where the "break" or "kase" serves not to release the tension but rather to climax it in a galvanizing shock, the first enormous blow of kase, which abruptly empties the head and leaves one without any center around which to stabilize. This is the state of helpless vulnerability. Instead of being able to move in the long, balanced strides of relaxation, the defenseless person is buffeted by  each great stroke on the drum, as the drummer sets out to "beat the Lwa into the head of the one getting possessed".

~as taught by Master Drummer Frisner Augustin, God rest your Soul in Ginea

In the images below, you'll see that I had the honor of playing an Ecstatic Dance, the dancer in the photo is experiencing exactly what Master Frisner would refer to as complete surrender of energy and spirit, allowing for what is known as "communal or trance possession". The energy of the drum and of the dance is extremely powerful and will take you to other levels of consciousness, what the Shamans call ASC (Altered States of Consciousness)