I think when they first introduced it, it had to be played before the vote and it was never used. Then I think they changed it to after the vote for one (or two?) season(s) and then changed it back.
Remember at tribal council when Jeff Probst says "Before" the vote, that if anybody has the immunity idol now is the time to present it.
Last survivor he asked if anybody had the immunity idol "after" all the votes had been counted. Remember that jet fighter dude had the immunity idol at the final four and it was a tie and it went to fire challenge that went on for over an hour an pissed jeff off so much he gave them matches? Well he could of avoided all that and just handed her the idol it was the last time it could have been used because in that last survivor it could not have been used after the final four and he held onto it which I thought was like totally crazy. He never made it to the final two because of it as well. This survivor they are making it much harder to use.
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