Prior to arriving at the idea of a sit-down strike, laborers who wanted to go on strike risked violent retaliation, replacement workers taking their jobs, and their fate resting in the hands of the union leaders to settle the dispute. In the early 30s, the "sit-down" meant a small number of workers could stop working, where they were, halting production, making scab workers impractical, strikebreakers risk damaging expensive machinery, and they protesting proletariat even got to stay out of the elements. When first implemented, they were very successful, and even fostered a greater sense of work unity, as they had the opportunity to converse more under these circumstances and create a greater sense of, might I say, comradeship.
Under threat of a wage cut in early 1936, employees at Firestone in Akron staged a sit-down strike which was settled in two days. Less than two weeks later, there was a sit-down at Goodyear. An injunction against picketing was ignored and 150 new deputies found themselves facing 10,000 united Akron workers. The strike was settled within the month.
In December came the longest sit-down strike ever, at Fisher Body Plant in Flint Michigan. The UAW damanded recognition from General Motors. The strike spread to other major cities, including Cleveland, and eventually over 70% of their plants sat idle. GM demanded the governor of Michigan step in with their militia and there was great fear of bloodshed. FDR eventually stepped in (not physically, of course) and the strike was settled in February, 1937.
In 1936 there were 48 sit-down strikes. The following year there were 477.
Howard Zinn, A People's History of the United States