Quick answer
Read the two exposures for the card section they point to, count what the player still needs, name the hot tiles, and decide whether to keep attacking or throw safe.
Two exposed sets on one rack changes the hand for everyone. That player has told the table most of what they are building, and the tiles that finish them are now knowable. Here is how to read the two exposures, how to find a safe discard, and how to decide whether your own hand is still worth chasing.
Read what the two exposures say
Every exposed set is a pung, kong or quint, so the hand is an exposed hand, not a concealed one, and it comes from a section of the card where those sets appear. Two exposures usually narrow it to one or two sections: matching numbers across suits point at Like Numbers, a pair of even pungs at 2468, a pung of dragons beside a numbered set at whichever section pairs them.
Look at the card with those sets in front of you and find the handful of lines they fit. You do not need to know which line exactly. You need the family, because the family tells you which tiles the player is still hunting.
Count what they still need
A hand is fourteen tiles. Two exposures account for six to ten of them, so the player is holding four to eight more, and nearly every line needs a pair somewhere. What is left to find is one or two more sets plus that pair, and each candidate line names them.
If the missing pieces are all pungs and kongs, jokers finish them and the player can be closer than the rack looks. If a line needs a natural pair, that pair is the bottleneck and the two tiles that make it are the hottest on the table.
Name the hot tiles
The hot tiles are the ones the candidate lines still need: the same number in the missing suit for a Like Numbers hand, the remaining evens or odds for a numbers hand, the dragon that matches an exposed suit, the flowers or winds a line calls for. Say them to yourself before you discard, every turn, until the hand ends.
Then check the table. Any hot tile with three copies already out is cold again, because no pair or pung of it can be completed. Any hot tile nobody has seen is the one to fear.
Find your safe tiles
A safe tile is one that cannot help any of the candidate lines: a suit and number they have no use for, a tile with three copies already visible, or a wind or dragon that fits none of the sections in play. The safest discard on the table is usually a tile of which three are already out.
Discards the player has already thrown are usually safe too, because a player rarely breaks up a set they were keeping to throw a tile they now need. Usually, not always: a hand that changed direction after a joker exchange is the exception.
Decide whether to keep attacking
If you are two or three tiles from mahjong, keep going, but throw the safest of your own spare tiles first and hold the hot ones until you have no choice. If you are four or more away, the hand is now a defensive one: the aim is to reach a wall game or let someone else feed the winner, not to finish.
East pays and collects double, so a player sitting East with two exposures is worth more caution, and a player who is East themselves should defend earlier than usual.
When to break your own hand
The moment your only remaining discards are hot tiles, break the hand. Throw a tile from your own keep set that is safe by the count above, even though it costs you the hand. Paying for someone else's mahjong costs more than a wall game, and it costs everyone else at the table too.
This is the discipline that separates players who win over a season from players who win a hand: the willingness to give up a decent hand rather than hand a better one to the rack across the table.
The tells that they are one away
A player who stops rearranging their rack, who discards without thinking, or who starts watching one particular suit go by is usually waiting on a single tile. So is a player who exchanged for a joker two turns ago and has been silent since.
Reading exposures is the skill that makes those tells useful, because the tell says when and the exposures say what. The coach names the hot and safe tiles for any table position it is shown, and a few dozen such reads make the whole calculation automatic.
Put it into practice
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The hardest part of defence is holding the whole count in your head while your own hand wants attention. Mei, our AI coach, reads your tiles with the table's exposures in view and names what is safe to throw.