Scooping!
All those bumps, lumps, back fat, armpit fat - they get yanked in to the bra! However it only works if A: the back is small enough and b: the cup is big enough.
Basically, put bra on, then stand side on to a miror. Lean forward, put your right hand inside your left cup, reach round as far as you can and pull/scoop/yank all the flesh/soft tissue that is bumping under or over the back, around the straps or the armpit and haul its ass in to the cup. Repeat at other side. You might need to do it a couple of times (as doing one side knocks the other out slightly)