Why inducing competency in E. coli using calcium chloride requires ice - (Jun/28/2006 )

I'm a student and there and I am using a protocol where competency is induced in E. coli using calcium chloride. Basically the protocol is about centrifuging the bacteria, resuspending the bacteria (pellet) in ice-cold calcium chloride followed by placing the mixture in ice for 20 minutes.

Is there any reason for the cold temperature and the placing of mixture in ice for 20 min?

Thanks in advance...

This link can help a bit: http://www.genome.ou.edu/protocol_book/protocol_adxF.html

Thanks. The link helped