if the ring is worn, the seal is no longer air tight, so the draw of water loses power.
http://www.homedepot.com/p/Fluidmaster-Wax-Toilet-Bowl-Gasket-7510/203763996?N=c6a1#
http://www.homedepot.com/p/Fluidmaster-Wax-Toilet-Bowl-Gasket-7510/203763996?N=c6a1#