As other alluded to, that's a spot promote, different from a frocking.
@HAL Pilot set me straight that frocking =/= pay. Spot Promote on the other hand does get you paid. SWOs spot promote for tough engineering DH jobs. I know Sub guys and NSW do spot promotes as well. For the spot promote, you just have to fill the specific job. You don't even have to be screened for that pay grade; this equates to some guys getting paid for O-4 18+ months earlier than they would be otherwise.