Is a gizmo, detailed in the accompanying link, which I think, does what it claims..
I listen to a fair amount of music via in-ear stylee earphones and the introduction of this little doodad has improved the experience.
I thought that I may have simply decided that the sound was different with / without it..and went for a scientific trial scenario, involving my mate Dave and the Hump album by Sopwith Camel – which if you don’t know it, you should ! We both agreed that the sound without the BH unit sounded inwardly driven whereas incorporating it made the sound erm, outward. ( Doubtless someone with more grey matter than Dave or I will explain this more coherently.)
Anyway, it’s well worth £15 – or 2 metropolitan elite pints..