to advance
兟 centers on moving forward or advancing, often with a sense of paired or repeated progression.
兟 is formed by doubling 先 (before, ahead), suggesting a reinforced idea of advancing or going forward. The exact historical origin is uncertain, but the reduplication pattern is a known way to intensify meaning.
Two 先 (ahead) side by side: imagine two people racing forward, each trying to advance ahead of the other.
For シン, picture the two 先 components as two shins (legs) striding forward: shin -> シン, and both shins advance together.