I had a hard time getting my head around exactly that when I was still flying gliders.
With competition gliders you have the option of carrying water in ballast tanks.
Purpose being to increase the "best distance" glide speed while maintaining the best glide ratio.
This article explains it well:
http://home.comcast.net/~verhulst/GB...t/ballast.html
Here is a pic of a glider dumping water during a finish pass:
