Yeah another person posted a thread about driving in M8 mode where both ICE and electric are running and he noticed some efficiency gains. I too noticed it but from a power perspective it felt similar to regular driving.You can sort of force this mode by bumping the gear shifter to manual mode and shift manually. The gas engine stays on but you have to deal with the tedium of manually shifting. The minute you put it back in "D" the engine will shut down asap.
Would be interested to drive in this fuel and refresh mode you are talking about. Do you know how to force that? Or it was just random?