Actually, they never did show the true flashback of how he did it. They showed many possibilities including one that Sherlock gave Anderson but Anderson questioned whether that was the true explanation and the show made it appear that Anderson was correct. At the end of that episode, Watson asks Sherlock what the true explanation was and the implications was that Sherlock gave Watson the true explanation but not on camera.

Apparently my original post has been disappeared. I didn't use any bad language or say anything provocative. I don't understand what the problem could have been. I will resummarize:

Elementary was way worse than Sherlock

1. Elementary made Watson the co-equal of Holmes likely for political reasons when in the books Watson is there to chronicle Holmes and occasionally provide muscle is somehow verboten. Watson's role is not supposed to be just the female version of Holmes and Watson is not supposed to be as smart as Holmes.

2. Johnny Lee Miller doesn't look or speak anything like Sherlock Holmes and I had to keep reminding myself that he was supposed to be Sherlock Holmes.

3. They made Holmes hypersexual when he is the opposite of this.

4. It became a boring police procedural very quickly.

5. The Everyone deus ex machina was ridiculous

6. Making Irene Adler the same person as Moriarity was for shock value which ruined both characters.