As it's moving 'away' from Earth, to an observor standing on Mars, it looks like Earth is moving 'backward' as this is happening. Mercury is surging ahead and Earth is left behind. This is 'retrograde motion'. When the engines are in 'forward' position, you zoom around. That's 'retrograde' motion.
Earth is the water in which it floats.
And this 'energy' is translated into 'retrograde motion'. You zoom past another boat that is anchored and the gal is fishing in one spot. You watch the waves your boat makes impact that other boat. That other boat will bob and move around as the current of the lake water, set into motion by your speeding boat, strikes it. The waves pass, and the boat that is anchored, becomes still once more.