Spiral Dives Explanation.

I've been looking for an expalantion of the aerodynamics of how an aircraft
gets into a spiral dive. Mostly I've found lots explanations of what the
signs
are and how to recover, but those are things I already know. I've gathered
that
it's related to lateral stability, and that the wings are not stalled, but I
don't yet
understand how it comes about that adding back pressure increases one's
airspeed.
If someone could point me to a good explanation of what's going on
aerdynamically
when an aircraft is in a spiral dive, I'd appreciate it.