The second pour encases the ductwork within a thermal mass of concrete. Warm water and an accelerant was used in the concrete mix due to winter temperatures.
The homes forced air system will pass a series of ductwork through the slab creating a "heat energy battery". In this image 8" round ductwork is being tied into the central trunk line.
A high compressive strength foam board was at point loads and bearing wall locations. The foil faced insulation board, which has a greater R-value was used everywhere else.