SWA offers open time to be bid on in seniority order. The pilot has the option to bid on the open time at straight pay or premium (150%). If the trip does not have straight pay bidders it will be awarded as premium pay. Lots of other ways to score premium also. Re-routes that moves your show time earlier pays premium for the whole day. Reassignment that gets you in later than originally scheduled pays premium for the legs past original release. Junior assignment etc.