Say heat conducts from body at 100 degrees through material A to interface at 50 degrees, further flowing through material B to extremity at 0 degrees. If you observe that during transfer, the interface at 50 degrees is heating up, it means that it wants to reduce transfer through material A and increase through material B, so that rates through A and B are equal. Similarly, if 50 degrees begins to cool down, it means material A is a poorer conductor than material B, hence it has to adjust itself so that differential through material B is reduced and differential through material A rises, to compensate for poorer conductivity, so that transfer rates are balanced. It works this way only in series conduction.