Zero-force members prevent a truss from becoming a mechanism -- Why?

[JakeMEng]

My professor explained to us that the purpose of a zero-force member in a truss is to increase the stability of the truss during construction, to provide added support if the loading is changed and to prevent the structure from becoming a mechanism.

It is this latter point that I do not understand the meaning of. What does it mean for a structure to become a mechanism? Why is this unwanted? Why does a zero-force member prevent this?

Thank you

 

[Simon Bridge]

You should get all your answers from the definition of "mechanism".

 

[berkeman]

Is it like racking?

 

[Simon Bridge]

Your responce suggests you do not know what "mechanism" means in engineering. You should go look it up.

(Mentor added link) https://en.wikipedia.org/wiki/Mechanism_(engineering)