A OnOff joint is a constraint for a selected translational or rotational degree of freedom of the action reference frame. Either of x, y, or z component of the action marker relative to the base marker is constrained. These constraints can be turned on or off by the specified function. Definition of a valid function expression is explained in Function Expression.
Figure 1 OnOff icon on Working Window
Figure 2 OnOff icon of the Joint group in the Professional tab