Which coach led their team to an improbable championship victory despite facing significant adversity and being considered a heavy underdog?

1. Hint 1: His coaching style is often described as unconventional and relies heavily on player empowerment.
2. Hint 2: He had a relatively short playing career and transitioned into coaching at a young age.
3. Hint 3: His team's championship run involved overcoming a significant deficit in the series.
4. Hint 4: He's known for his calm and collected demeanor even in high-pressure situations.