When can you cut in in a staggered start for a 400 meter race?

In a staggered start for a 400-meter race, runners can cut in after they have completed the first turn and entered the straightaway. This typically occurs about 100 meters into the race, once the runners have passed the point where the lanes converge. The specific cut-in point is marked on the track, ensuring that all athletes have a fair opportunity to compete.