What's the purpose of partial autopilot disengagement after applying certain force to the control wheel? Aeroflot Flight 593 crashed after the autopilot was partially disabled by applying certain force to the control wheel but the only function which was disabled was controlling the ailerons. This partial disengagement went unnoticed for a number of reasons.

I'm reviewing the official accident report (text is in Russian, linked to from the Wikipedia article) and here're my findings from there so far.

The flight manual (cited in the report) is claimed to say that manipulating the control wheel to contradict the autopilot is definitely abnormal and should be avoided. The mechanism for overriding the autopilot is a safety device that engages outside normal operational conditions.

So it looks like there some functionality that allows to override the autopilot and partially disconnect it (so that only ailerons are no longer controlled) and it is claimed to be a safety measure.

What's the purpose of this functionality?
The autopilot will disconnect on large control inputs because this is the most intuitive way to control the plane. On Boeing planes, the controls will physically move along with the autopilot. If the plane starts doing something you don't want it to do, the instinctive action is to grab the controls to correct it. While the autopilot button would normally be the way to disconnect the autopilot, it may not be the instinctive way in an emergency, and it can be helpful to already have both hands on the controls when the autopilot disconnects, rather than having one hand disconnecting the autopilot.

The reason for only disconnecting one part of the autopilot could be to help the pilot. Maybe there is a problem with the elevator controls, or an upset that affects the aircraft primarily in pitch. Maybe the TCAS gives a "climb" command. The pilot wants to take over pitch control to address this situation, but other controls are fine. While the pilot now has control of just pitch, the autopilot is still taking care of the other controls. The autopilot only gives the pilot control of the modes that are being overridden, instead of just turning off and requiring the pilot to deal with all control modes.