Read the following passage
The grandfather clause was a provision in some southern states that allowed illiterate whites to vote because their ancestors had been able to vote before the Fifteenth Amendment was ratified, effectively bypassing literacy requirements for white citizens while denying them to Black Americans.
In the context of the Voting Rights Act of 1965 and the struggle against Jim Crow laws, how did the 'grandfather clause' function as a mechanism to undermine black voting rights, and why was the Voting Rights Act significant in addressing this specific issue?