Just (now/then); as soon as — used to indicate that one event happens immediately after another (often in the form 'V-ta とたん（に）').
ドアを開けた途端、猫が外に飛び出した。
The moment I opened the door, the cat darted outside.
彼が電話を切った途端、また電話が鳴った。
No sooner had he hung up than the phone rang again.
雨が降り出した途端、祭りは中止になった。
As soon as it started to rain, the festival was canceled.