from Volcar - to tip-over or capsize;
a falling step: The leader causes the follower to tilt or lean forward and fall off her axis before he catches her again. The process produces a beautiful leg drop from her. The movement requires the support of a close embrace.
from tejastango.com (http://www.tejastango.com/terminology.html)
In a close embrace, one leads the woman to "fall / lean" on you, through going backwards, so she capsizes off her axis, leans and drops her knee, making circles with the foot.
from Tango-E-Vita (http://users.pandora.be/Tango-E-Vita/tangoiste/tangolessen/volcada.htm)