expression, ichidan verb
synchronize breathing with the opponent before a sumo match
Sumo term: both wrestlers coordinate their breathing rhythm just before the initial charge (tachi-ai).
両力士は呼吸を合わせてから立ち合った。
Both wrestlers synchronized their breathing before charging at each other.
Compound of 呼吸 (breathing) + を + 合わせる (to match, to synchronize). The phrase is specific to sumo.