As the name suggests, the 'Pill of Hunger' makes one starve.
In the first part of 'Dao of Heart', the technique requires one to steel his heart as it gets smelted with the fire of hunger.
Hunger is the reason the world moves. Be it men or those spirit beasts, the only reason they even bother to fight comes from hunger.
The hunger that doesn't discriminate against anyone. As long as one is hungry, he would do anything to satisfy it.
As a technique learned by Buddhists, they must conquer this most primitive instinct of men.
Once one learns to control his hunger, he would be able to make the calmest decision. Be it hunger for food or hunger for power, he will be able to reach your goal most efficiently. That is the ideology behind the first part of the 'Dao of heart'.