Satyagraha - is the name of Gandhi's form of Nonviolent Resistance. It influenced Martin Luther King Jr. and James Bevels' Civil Rights Movement in the US.Satyagraha means - truth force. Satya (Truth) and Ahimsa (Compassion) are the core values of Satyagraha. Satyagraha is not just a tactic for political struggle, but a universal antidote for all injustice and harm.
Principles
There are 3 responses to injustice:
1. The coward's way of running away (which essentially enables wrong to continue)
2. Violence
3. Non-violence
Gandhi says we should not use violence under most circumstances, except as self defense for invasions or private property (stealing, breaking into your home etc.) For most instances it's best to restrain yourself, and not use violence at the slightest pretext. Therefore he ruled out all forms of direct coercion, like trying to block someone, using hostile language, destroying property. He did so because it undercut the empathy and trust the movement was trying to build.
Gandhi never wanted to overthrow or defeat anyone. The goal is not to cause division or frustrate opponents, but to help transform and purify them. If it worked, both sides won. All Gandhi's actions came from a love of humankind. The love for the victim demanded struggle to speak up, and love for the opponent ruled out bringing harm to them.
Gandhi is able to sympathise with oppressors because he acknowledged that by hurting others, the oppressor hurts himself. The oppressor might be unaware, and be thoroughly enjoying his power and wealth. But what happens is the injustice he creates shuts himself off from fellow humans, and soon his spirit withers.
What Satyagraha does
The common practices of satyagraha were non-cooperative: such as refusing to cooperate even when opponents strike, illegal non-violent protests, economic boycotts etc.
Only specific unjust laws were to be broken, you shouldn't just blindly flout laws. Only people with high regard for law, qualify for protest, because no one would think much when a law is broken, if people didn't care for it.
Before using Satyagraha, people must undergo training to ensure discipline. Gandhi founded an Ashram to teach Satyagraha. In it, he taught the value of truth - meaning both honesty, and living in full accord to what's right. He also taught other important ideals like equal respect for all religions, fearlessness, self-control, not insulting your enemies, and economic strategies like boycotting imported goods.
How Satyagraha was Effective
Because the power of any tyrant depends entirely on people willing to obey, an amazing thing happens once people say they're not afraid of prison and are willing to die. No government can exist for a moment, without the cooperation of people.
Gandhi wanted to make the statement: that he cared so deeply about his cause that he was willing to undergo legal penalties. And from his depth of concern, he's bound to change people's minds and see the truth of his cause.
Gandhi and his followers would joyfully deal with face beatings and imprisonment. Members of the public would be impressed by the protest. The public would put pressure on their leaders, until the politicians eventually gave in.
Impact
Gandhi didn't invent nonviolence, but he raised it to a level never before achieved. He was the 'discoverer', in the sense that he was the first to have a general concept of non-violence, declare it, and then consciously apply it in his works (similar to how Newton was not the first to discover an apple falling from a tree, but was first to grasp its significance and answer 'why').
Martin Luther King Jr. in his autobiography, said he was skeptical of the power of love in revolt. But after visiting India, he learnt how important love was, and discovered the method for social reform he had been seeking.
To quote: 'I left India more convinced than ever before that nonviolent resistance was the most potent weapon available to oppressed people in their struggle for freedom'
Sources:
Commentary on Gandhi's Nonviolence:
http://www.markshep.com/peace/Myths.html
https://www.mkgandhi.org/faq/q17.htm
Wikipedia Page on Satyagraha: