This is just improvisation. Nothing new. Any light colored card/paper etc & for this instance aluminium foil is used, can be used to reflect/fill light.
I thot you guys should know this already? :dunno: :think:
I'm using a foam paper to use as a bounce card ... 2 pieces, 1 white and 1 black glued together, with the white side facing the flash/subject. Cut it such that the top part not attached to the flash is flat and wide, and the bottom part wraps the flash on 3 sides. To hold it, can use velcro straps or worst come to worst, rubber bands.