How did Gandhi's nonviolent movement work against the British?

Gandhi's nonviolent movement for Indian independence from the British put a spotlight on British injustices towards the Indian people and gave Gandhi the moral upper hand. The British could not accuse the Indians of being violent instigators or behaving in uncivilized ways that justified retaliating with force when their protests were based on nonviolence. Often the British, whose colonization of India was based on military force, were backed into difficult situations by the nonviolence. They could and did round up and imprison ringleaders, but they couldn't stop the movement for independence. Step by step, Gandhi turned world opinion in favor of Indian independence and against British rule.
Integral to Gandhi's notion of nonviolence was the concept of satyagraha. Satyagraha roughly means the force of truth and love. He understood it as very powerful but nonviolent force. For Gandhi, it was more than "passive resistance," as the word "passive" suggested weakness. Gandhi did not conceive of nonviolence as passive or simply as taking meekly whatever was doled out. He understood it as taking nonviolent action against injustice.
The Salt March in 1930 was an example of satyagraha. Gandhi educated his followers not to behave violently, even if people tried to push them into violence. When they were ready, he had them march to collect salt from the sea, an act that was illegal under British law. The British had decreed the Indians had to buy salt from them, when they had been accustomed to gathering it for free. For a very poor population, having to buy salt was an economic burden. Gandhi wanted to publicize to the world the unjust way his people were treated—and he did so, shaming the British. Eventually, his methods of nonviolent protest were an important factor in India's gaining of independence.