The other day I rode a 5 year old gelding (trained to run barrels) that was so sensitive, you barely touch his side with your knee and he would move. He was so light, you didn't need to hang on his mouth or anything, just squeeze your bum muscles and he would stop.
I know this is a vague question, but how did you knowledgeable people out there start teaching your youngsters to rein? What measures did you take, cues, ect to soften them up so much? I mean the softest touch causing a sharp and quick response?