Use the Spring Option dialog to determine how springs are added in the Flex modifier when you click the Advanced Springs rollout 
Add Spring button.
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Select a Mesh, Patch, or NURBS object. >
Modify panel > Modifier List > Animation Modifiers > Flex > Advanced Springs rollout > Options button
Interface
- Single Edge Spring
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Creates one edge spring between two selected vertices. If any number of vertices is selected other than two, no springs are created.
- Hold Edge Length Springs
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Creates edge springs along the edges of the objects between any vertex selection and neighboring vertices.
- Hold Edge Length Springs Apply Only To Selected
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Creates edge springs along the edges of the objects between all selected vertices.
- Hold Shape Springs
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Creates shape springs from the selected vertex or vertices to all other vertices within the Hold Shape Radius.
- Hold Shape Springs Apply Only To Selected
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Creates shape springs between all selected vertices within the Hold Shape Radius.
- Hold Shape Radius
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The radius within which shape springs are created. No shape springs are created between vertices farther apart than this distance.
At the bottom of the dialog is an informational display showing the object's average edge length, maximum edge length, and minimum edge length. This information can help in determining an appropriate Hold Shape Radius setting.