These dissimilar training innovations began in the 1969, when the U.S. Navy established the original "Top Gun" fighter pilot school. This was done in response to the poor performance of its pilots against North Vietnamese flying Russian made fighters and using Russian tactics, with some Vietnamese innovations added. What made the Top Gun operation different was that the training emphasized how the enemy aircraft and pilots operated, thus the term "dissimilar training".
Before the 1969 innovations American pilots practiced against American pilots, with everyone flying American aircraft and using American tactics. It worked in World War II because the enemy pilots were not getting a lot of practice and were using similar aircraft and tactics anyway. Most importantly, there was a lot of aerial combat going on, providing ample opportunity for on-the-job training. Not so in Vietnam, where the quite different Russian trained North Vietnamese were giving U.S. aviators an awful time. The four week Top Gun program solved the problem. The U.S. Air Force followed shortly with its Red Flag school. In the early 1980s, the Russians established a dissimilar air combat school, and the Chinese followed in 1987.
The U.S. developed the ACMI pods in the 1970s, and they have undergone continuous improvement ever since. The pods collect and transmit to the ground ACMI station aircraft information, which includes speed, altitude, attitude, current G-forces, ascent/descent rate, turn rate, yaw rate, roll rate, engine power, missile cue, rocket cue, gun cue, bomb cue, and weapons release points. This last item allows the ACMI to calculate the range, trail, and heading of each weapon. In air combat this would mean simulated gunfire or use of air-air missiles. About the time the Chinese began developing their ACMI, the U.S. began developing methods to do away with the ACMI pod and use the internal data systems on the aircraft to do what the pods had been doing.