The hump is a waterproof Hi-Viz cover designed for use with rucksacks of capacities between 20 and 60 litres.
One size is pretty much universal - even on smaller bags (although not on the older, smaller hydration packs), the elastic ensures a secure fit, eliminating infuriating flapping even in blustery conditions. And fitting couldn't be simpler: roll the hump over the bag, secure the elastic straps in a cross formation and pop the press-studs together.
It seems just as happy fitted to panniers, although really struggled when faced with a 60-litre expedition model. In fairness it still fitted and stayed that way through winding lanes and strong headwinds (although I stopped periodically to check the straps were secure).
Competent at both tasks, the hump is a sensible upgrade for most bags and protects them from road grime and spray. Its yellow burns the eyeballs nicely, even in low light, and the big sections of Scotchlite shout your presence to other road users after dark.
Packing down small enough to fit in a jersey pocket, it makes an excellent companion, not only in the depths of winter but late spring evenings where lights get forgotten and an untimely puncture can see you out later than planned.