what is back to back LC

You received an L/C, a master L/C, from your customer. You use this L/C to back up and and ask your banker open a new L/C to your supplier with nearly the same terms of this incoming L/C. Talk to your banker for detail arrangement e.g. deposit etc.
http://www.businessdictionary.com/definition/back-to-back-letter-of-credit-L-C.html
Details in
http://zhidao.baidu.com/question/19750996.html