The resistance band reverse fly primary works the trapezius muscles, located at the top of the back, with the triceps and also forearms worked throughout this exercise as they stay contracted throughout the movement. I would normally complete this exercise with a light resistance band, as you will be extending and stretching the band extra for this movement lighter bands are good to start with them increase the tension gradually. 

Method 

Starting in the kneeling position the resistance band underneath both knees, holding each end of the band in each hand, move backward until your arms are extended in front of you and you feel some tension in the band.This is your starting position. 

Pull the bands out and to your side as far as you can, keeping your arms extended, before exhaling and returning to the starting position.