Knowledge about similar objects out at this distance (Pluto, Charon, Enceladus) have been used together with laws of angular momentum to work out what is physically possible. It ignores formation mechanisms at this point so as to concentrate on what is physically and mechanically possible given observations of Haumea and several other well observed analogues.
Pluto, Charon and Enceladus have been found to have ice shells and liquid water oceans beneath. Geysers on Enceladus go to form a ring at Saturn, so this probably explains the formation of Haumea's ring. Out-jetting of water at speed is what is happening at Enceladus, and this would be enough to torque even such a large body as Haumea. A thick ice shell would be enough to hold up to compression at the neck as the centrifugal force, friction, and conservation of Angular Momentum does the rest.
There will be gradual and even stretching of the neck as angular momentum is increased - in the same way as couples ice-skaters do the "spiral of death"
Tidal friction from the moons and ring keeps the motion circularised in the same way as the skating spin.