A pit or trapped area, approx. 2' deep and 3' wide, running along the DS side of the cyclorama specifically for the purpose of lighting from the bottom--to house the cyc footlights or electrics ground row. Obviously this is something that needs to be designed at the time of contruction, as it is not easy to retrofit a cyc pit.
A vertical section drawing:
http://www.rosco.com/us/technotes/screens/technote_tbcs.cfm
A vertical section drawing:
http://www.rosco.com/us/technotes/screens/technote_tbcs.cfm