A blow bag is a device used for drain-cleaning, it is made of a rubber bladder with a hose fitting and a nozzle on the other end. The blow-bag needs to be attached to a water hose and inserted into a clogged drainpipe. When the water is turned on it will fill the blow bag and it expands gripping the pipe. The blowbag releases pulsating bursts of water through the nozzle forcing the water to clear the clog.
Ehow.com: How to Use a Blog Bag