Wafter, the latest name for an extremely critically balanced bait
Seriously you could argue semantics, but to me a wafter should sink, but extremely slowly of its own accord, or have a neutral buoyancy, it would stay in what level of water it was put in.
If it doesn't sink, but sits on the surface, then it is a pop-up!
That slow sinking bait is going to be lighter than the freebies, and will be more easily sucked in than the free baits, and the hook will follow it in due to the hair.
A bait that sinks with the weight of the hook, but would normally float, is a pop-up, even if the weight of the hook overbalanced it.
Various hooks have different masses or weights, so a pop-up that is sunk by a size 6 Mugga is more likely to pop-up a size 10 101.