Short version of how it works: in mahjong you normally draw one tile, and then discard one. You can see your opponents discards, but obviously not their draws.
In Duel of Divination mode, you get to choose one of three tiles as your draw, pass the other two along, and discard as usual. These three tiles are always known to the table. Your opponents know your draws. Meaning, if you're big-brained, you can figure out exactly what your opponents are playing toward.
I am not big-brained. I have a hard enough time guessing off of discards. Trying to keep track of three players' hands while building my own is going to kill me.