bottom pipe

See http://www.controlbooth.com/forums/glossary/10940-pipe-pocket.html. Steel pipe, most often 1/2" schedule 40, up to 21' lengths, that is threaded on both ends and connected together via pipe couplers are fed into the pocket and is used to keep the drop tight and weighted down. If the backdrop is short and doesn't need much tension, EMT or PVC pipe may be used instead.