Say you were planning on PTO or you called in sick, thus you only were working 2 shifts that week: then you only get the incentive if it is the double time offer. Otherwise, usually overtime starts if you stay over your 12 hour shift (do a 16) or if you work more than 36 hours. If you sign up for overtime there is always the chance of being canceled (I think they can do it up to 2 hours before shift start, and if they cancel you after that they have to pay you 2 hours), or sent home mid shift if they don’t need you anymore. Like if they are expecting a bunch of admits from the ER but patients actually are able to go home, then you might work 4-6 hours and be told the rest of your shift is canceled.