Complex sentence: The Rattlers then officially left the AFL for the IFL on October 17, leaving the AFL with four teams and the the sixth AFL team to leave for the IFL since the 2010 relaunch.
Split into simpler sentences: The Rattlers then officially left the AFL for the IFL on October 17, leaving the AFL with four teams. They were the sixth AFL team to leave for the IFL since the 2010 relaunch.