One of the basic belief of nonviolent direct action is to make the oppressor face the difference between the oppressors ideas and the oppressors actions. how did selma march use this principle

The march on Selma used this principle by using nonviolent means to get an idea across to the people and to the authorities. The protesters merely marched, and when they were met with violence in the form of beatings, the public saw this and was awakened as to the ideas of the march and the people marching. This was seen in contrast to the oppressor's (authorities) actions.