How does the elastic clause keep the Constitution "alive"?

The elastic clause allows legislatures to make new laws based on Constitutional principles as situations change.

http://www.sparknotes.com/history/american/statebuilding/terms/term_10.html

Examples of checks and balances: Congress makes laws, but the President can veto them.
The Supreme Court can rule a law unconstitutional.
The President recommends Supreme Court justices, but Congress must approve them.