Edit Curves > Modify Curves

When you select Edit Curves > Modify Curves > Lock Length, the selected curve (or selected CVs) will maintain a constant hull length. If you then modify the curve, for example, by moving a CV, the shape of the curve will adjust so that it maintains a constant hull length (that is, CVs other than the one you move will also move).

When you select Edit Curves > Modify Curves > Lock Length, a Lock Length attribute is added to the curveShape node.

Notes

You can use the l (lower-case L) key as a momentary hotkey. Press and hold down the l key to lock the length of the selected curve. Release the l key to unlock the length of the curve.
Lock Length is useful for modeling purposes. Because modifying a curve with Lock Length turned on behaves similar to a dynamic simulation, animating a curve in this way (with keyframes) may produce unexpected results.
If you turn on Lock Length, move a CV and then select Undo, only the CV you moved will return to its original position.
If you turn on Lock Length and modify the curve, the curve’s first CV will not move (unless you explicitly move the first CV). For example, if you move the curve’s first CV, the curve will respond differently than if you move the curve’s last CV.