Rebbe Moshe Kobriner zy'a in Toras Avos writes:
Rashi explains that Moshe Rabeinu had difficulty understanding what was meant until Hashem showed him a vision of a fiery coin.
What was so hard for Moshe to understand that Hashem needed to show him a fiery vision?
Moshe's question was how was it possible for a small coin to be a kofer nefesh: an atonement for a person's soul? A person would give everything he has to preserve his life!
Hashem showed him the fiery coin which symbolizes the fiery force of the Yetzer Hara, who works to keep us from giving even a small thing away. That is why even a small coin can be a kofer nefesh.
With every 'urge', we are struggling against the mighty force of the fiery Yetzer Hara.
Even a small step away from lust can be a Kofer Nefesh!