Checks and Balances: This system basically further enforces the restrictions set by the separation of powers. It also allows another branch of government to restrict the other in order to prevent a situation in which one branch is trying to take control.
Ex: If a law is passed by Congress in the legislative branch, the President has the right to veto the law. It then goes back to Congress and if 2/3 of the people from both the Senate and House of Representatives are in favor of this law, it will be passed.