That one wasn't clear to me either, maybe as I skipped all the tutorials. I'm fairly sure though that it relates to the two scenes where you have to match the characters (represented by circular portraits) to the evidence/statements. The first one relates to alibis and resulted in you deducing there was conflicting evidence about when the wife supposedly left the island.

I got the achievement even after making mistakes in both so the timing isn't particularly tight but they are covered in the walkthrough if youwant the answers.

www.bigfishgames.com/blog/walkthrough/path-of-sin-greed-strategy-guide-hints-cheats-codes-tips-tricks/#2

Ah, thank you, that's very helpful. I guess I'll have to go through the game again, but with skipping puzzles and HOG hints it won't take long, as I got all the other achievements already.

Actually it unlocked for me not after the second time when you have to put together motivations and people, but when you have to describe how the second murder takes place (similar to the analysis of the first one, choosing from 3 options of what happened between the murderer and the victim). The sequence comes after you find the murder weapon (the circular marble labyrinth game) and your colleague asks you "what did you find?" in a muffled voice because he is some distance away.