It's a long piece of woven nylon that goes underneath BOTH spring-bars so in order to loose the timing unit both bars would have to break. They have military heritage hence the name, the most famous of course is the "Bond" strap from Goldfinger though that is technically a MoD G10 or "Zulu" not a NATO. A true NATO strap has an extra loop of material to lock the timing unit into a fixed position on the strap.
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