The 'Spring' is a special constraint the mimics the motion of an actual coiled spring. The Spring constraint produces a motion similar to the 'Point Constraint' swinging and swaying, with the addition of the stretching and compression action of a spring. It is represented in the viewport by a coil that will extend and compress as objects move during a simulation.
To apply a Spring constraint automatically, users can select two items prior to invoking the Spring constraint option. The first item selected represents the master or parent item, typically this is the fixed or keyframe animated item. The second item selected will be the item that is constrained, also considered the auxiliary or child item and will be constrained to the motion of the Spring. With the items selected, LMB+click on the 'Spring' button in the recoil toolbox, adding the Spring item to the 'Items List'. By default, the spring will stretch between the center points of the two selected objects. While Constraints can be added manually to a scene by way of the item list, automatically assigning them to items is the preferred method as it will also automatically define the constraints position relative to the selected items.
Name: This data field displays the current Spring items name. Users may easily change it by LMB-clicking within the field and typing the new name.
Spring--
Enable: The 'Enable' option toggles the Spring item on or off. When enabled the Spring will be considered during a dynamics simulation, when disabled, the Spring item will be ignored. However, disabled items are persistent across modo sessions being saved with the scene, and retain their present settings.
Stiffness: The amount of force it takes to compress the spring (shorten its length). A stiffer spring is harder to compress.
Damping: During a simulation, a spring will oscillate between extended and compressed. The 'Damping' effect reduces the amplitude of the springs oscillations.
Rest Length: The 'Rest Length' represents the length of the spring when it is not in motion. This can also be considered the distance between the two constraining items when the spring reaches equilibrium.
Breakable: The 'Breakable' option allows the Spring to literally be broken apart when a certain stress threshold is reached, defined by the 'Break Stress' value. When the threshold is reached, the Spring will no longer have any affect on the constrained item, leaving it to be affected by any other forces present in the simulation.
Break Stress: The 'Break Stress' value determines the threshold when the Spring will no longer affect the constrained item. The 'Breakable' option must be enabled for this value to have any affect on the Spring.
