the.com/starting gate

means The mechanical stalls that line up racehorses and spring open simultaneously so every horse starts the race from the same instant and position.

from Straightforward compound of 'start' and 'gate' — the device literally gates the start. Mechanical starting gates (stalls with spring-loaded front doors) came into racing use in the early 20th century, replacing older methods like a rope or webbing barrier or a starter simply waving a flag, because those earlier systems were notoriously bad at getting jittery horses off evenly.