1. A method of operating a system of linked gaming machines in which an award is given based on an award criteria, said method comprising the steps of: generating during a period of time one or more combinations of symbols in one or more of said linked gaming machines; determining, based upon said one or more combinations of symbols generated during said period of time, whether a first criterion has been met for making a feature available to said linked gaming machines, said feature, when available to said linked gaming machines, operating to alter said award given by said linked gaming machines; making said feature available to said linked gaming machines after it has been determined that said first criterion has been met; generating one or more combinations of symbols in one or more of said linked gaming machines after said feature has been made available; comparing said one or more combinations of symbols generated after said feature has been made available to a predetermined award criterion to which said feature applies; giving an award by one or more of said linked gaming machines based upon whether said one or more combinations of symbols generated after said feature has been made available matches said predetermined award criterion; and making said feature not available to said linked gaming machines after it has been determined that a second criterion has been met.