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The two strokes are never cross-faded and the icon is never swapped: the minus is the plus with its vertical stroke scaled to nothing, which leaves the whole move as one progress value driving two properties. Rotating is what supplies the motion — a bar that merely sprouted a second bar in place would read as a glyph being edited rather than an object turning — and a plus is unchanged by a quarter turn, so the rotation costs nothing at the far end and exists purely as travel. Halfway through, the grown-but-still-short stroke crossing the turned horizontal is the diagonal cross the move is recognisable by, and it falls out of the construction rather than being a keyframe anyone placed. Angle takes the spring's raw output and stroke length a clamped copy, because angle is the one quantity here with room to ring: overshoot past the quarter turn is visible and reads as the icon settling, whereas a stroke that overshot its length would simply grow longer than the icon it belongs to. Ring and fill trade places on that same clamped value — the border fades out from underneath the fill arriving over it — so the disc changes state once instead of stacking two changes that happen to coincide.