About the Update Positions Command

This command is typically used to impose joint constraints on a mechanism, after performing some operations on the assembly component of the mechanism.  You also use it after deleting one or more constraints from the specification tree.
  When you click Update Positions , the Update Mechanism dialog box is displayed.
 
  You can select a mechanism from the Mechanism list.  If you select the Take current positions for rigid joints check box, the current position of the rigid joints replaces the previous one in the constraint definition.