The asthenosphere is a layer in the Earth’s upper mantle between roughly 80 km and 250 kilometer depth. This particular region has a lower viscosity than either the region above (the lithosphere) or the deeper mantle below roughly 200-300 km (the lower part of the upper mantle). This is because the temperature of the asthenosphere comes closest to the melting point of the rocks there. When a solid approaches its melting point it becomes ductile although it is still solid.

After reading up on it to refresh my memory, the best answer that I can give you is that I don't think we really know yet. The lower mantle is denser, more viscous, and apparently more uniform in its composition and properties than the upper mantle, given the behavior of seismic (earthquake) waves traveling through it.

I don't get the impression that much more than that is known: for example, it's in contact with the core, which is molten metal. How much of the metal from the core gets up into the lower mantle? Could this be responsible for some of the lower mantle's properties, or is it purely a function of pressure? What effect does temperature have on all of this?