Why this is the skill that matters most
Every tile that leaves a hand is a fact. Stack enough facts and you can see dead tiles, which opponent is collecting what, and what is safe to throw — without ever peeking at anyone's hand. Strong players keep a running log: who · what · which discard number.
This trainer shows you a stream of discards, each tagged with the seat that threw it. Then it asks you to recall: how many of a tile appeared, who discarded the most of a suit, what the Nth tile was, whether a seat ever threw a given tile, and who made the most recent discard of a tile. Turn on reaction mode and a quick question interrupts the stream so you practice remembering while tiles keep coming. At the higher stages the pile fades as it grows, and the positional question drops out, because by then the only thing worth carrying is the running tally.