Assembly 3 Workbanch FreeCAD - Understanding the Lock Angle property

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QUESTIONS: 

 

1- Why is it necessary to change the Lock Angle flag to "True" to change the Angle property of a PlaneCoincident Constraint (applied to a part)? When doing this I notice in the Report Review that all the degrees of freedom (DOF) of the piece are zeroed.

2- Should the Lock Angle flag be set to "True", should the piece not become static, that is, without the possibility of changing the rotation angle?

3- I notice that the property "Lock Angle" is used a lot in Algebraic Expressions and Algorithms for animation in Assembly3. What is the reason in these situations?