TWI is a training program and methodology developed by the U.S. government during WWII. It was used to give supervisors key skills for leading a green workforce that replaced "the boys" sent to battle. Three key training programs are:
Job Instruction (JI) - taught supervisors how to effectively train their workforce
Job Relations (JR) - gave supervisors important leadership skills for dealing with people
Job Methods (JM) - taught supervisors how to break down a job and look for improvements

TWI was introduced to Japan during post-war rebuilding. It was adopted by Toyota and is, to this day, woven into the fabric of the Toyota Production System. JI is being used with only minor changes from the original. JR is credited with Toyota's "no blame" culture. JM was used and eventually grew into Toyota's Kaizen program.