Why a metal that springs back perfectly can still be permanently bent a moment later.
Push lightly on a steel ruler and it flexes, then snaps right back the instant you let go — as if nothing happened. Push harder, past some threshold, and it stays bent. Same material, same action, two completely different outcomes. The dividing line between those outcomes isn't vague or gradual — it's a specific stress value called the yield strength, and which side of it you're on determines whether the deformation is temporary or permanent. Most real forming operations, though, don't cleanly land on one side or the other — they involve both at once, and missing that is a genuinely common, genuinely costly mistake in metal-forming design.