It's hard to explain, but in physics it is known as
hysteresis, which means that the current state of a physical system depends not only on the current forces or factors applied to the system, but also on the past recent history of the system.
In a linear system, the system will respond in a linear way to the forces or factors applied to it, and once those forced are removed, the system will always return to its original starting state, it's natural point of equilibrium.
But in a system subject to hysteresis, even when the forces or factors are completely removed, the system may not return to its original starting state, but may remain "stuck" in some other state, because the system can have more that one natural equilibrium point that it settles into.
Some mathematical models of the HPA axis suggest the HPA axis could be a system subject to hysteresis, rather than being a linear system. If true, that means that when a significant factor acts on the HPA axis, once that factor ceases to act and is no longer present, the HPA axis may not settle back into its original starting point of equilibrium, but may remain stuck at some other secondary point of equilibrium. In which case, the HPA axis may not function properly.
So that's the idea of a HPA axis reset: to knock the HPA axis back to its original equilibrium point, so that normal functioning resumes.
But whether those HPA axis mathematical models in which hysteresis is found accurately reflect the real biological functioning of the HPA axis is another question.
But my guess is that if the HPA axis is not functioning from the basis of its natural state of equilibrium, and is instead stuck at some secondary point of equilibrium, there is likely some agency (like infection or autoimmunity) causing and sustaining this abnormal equilibrium, rather than this resulting just from hysteresis. But that's just my guess.