Like many sea creatures that can live in both deep and shallow waters, fuchsia bloods have a special organ for controlling buoyancy. This is done by filling it with gas when they want to float up. Due to the bipedal structure of troll anatomy this organ is housed externally on the chest. What I am saying is Fef has the ability to inflate and deflate her bobos at will. Seadwellers refer to them as "buoyancy bosoms"
My face when I saw this ask: