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

Give two examples of checks and balances.

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.