MS Dhoni's promotion ahead of in-form Yuvraj Singh in the World Cup, 2011 final remains one of the former captain's most successful and most discussed decisions to date. 

Today, it's easy to see the thought process and hence it does not shock us anymore but back then on the night of the final, it was perplexing. We were all asking why he did that and how.

World Cup 2011
Source: DNA

All these years later, we may finally have the answers. In a piece written for the Indian Express, India's former mental condition coach Paddy Upton mentions:

He was behind the full glass frontage that is there at the Wankhede dressing room. Gary was sitting outside and I was right next to him. I remember distinctly, I heard a knock on the window. Gary and I turned around the same time. It was Dhoni, he indicated he was batting next. That was it. Sign language. Gary just nodded.
Pady Upton
Source: Scroll

While adding that Yuvraj had nothing more to prove, and this is something Dhoni had an awareness of. He wanted to go out there - and play - and win. 

There was no talk between the two. Dhoni had made the decision that this was the time for him to stand up and do what Dhoni does. He would do what he is the best in the world at – which is seeing a team home in a chase in the second innings in a white-ball game. He had delivered nothing in the eight games before the final. Yuvraj had done his bit, he had played his tournament. He was done, he was spent. That moment was set up for someone like Dhoni.

Upton also mentions that the understanding between the captain and the coach was such that no words had to be spoken during that exchange. 

Dhoni had decided what he was going to do and indeed, so did everyone else. 

The moment is just a testimony to his leadership and courage, it spoke a lot about his relationship with Gary. Mind you, Gary didn’t have to stand up and have a conversation with Dhoni to discuss the merits and demerits of the move. It was just the two leaders of the team being on the same page. The knock on the glass, him pointing to himself, Gary’s nod… and it was done. I do remember very clearly when Dhoni walked down the stairs, I turned to Gary and said “Do you realise Dhoni is going there to fetch us the World Cup?” I had absolute conviction that Dhoni would come back with the trophy.
Dhoni and gary kirsten
Source: India TV

Legendary stuff.