starting gate; starting stalls
Used in horse racing to refer to the mechanical gate from which horses start a race.
発馬機が開いて、一斉に馬が飛び出した。
The starting gate opened and the horses burst out all at once.
Compound of 発 (hatsu, 'start') + 馬 (ba, 'horse') + 機 (ki, 'machine'). The reading はつばき follows standard on'yomi.