A deal-in, in Japanese houjuu (放銃), is discarding the exact tile that completes an opponent's tenpai hand. That player then declares ron. Outside pao liability on certain yakuman, you alone pay the full hand. Under Tenhou rules, a different liable player splits the full ron value with you; under WRC 2025, the split only covers the portion of the yakuman that was actually fed. If you are also liable, you pay the relevant amount alone.
The ordinary case happens on a discard. A kakan robbed by chankan is also paid by its declarer, subject to the same pao exception. An opponent’s self-draw is normally tsumo split among all three opponents, not a deal-in.
To reduce risk against one player, their genbutsu are certain against ron; suji and kabe remain probabilistic. Push or fold depends on value, shanten, safe tiles, turn and score.