The second Amendment of the proposed Bill of Federalism is a limit on Congress’ powers under the Interstate Commerce Clause…

The power of Congress to make all laws which are necessary and proper to regulate commerce among the several states, or with foreign nations, shall not be construed to include the power to regulate or prohibit any activity that is confined within a single state regardless of its effects outside the state, whether it employs instrumentalities therefrom, or whether its regulation or prohibition is part of a comprehensive regulatory scheme; but Congress shall have power to regulate harmful emissions between one state and another, and to define and provide for punishment of offenses constituting acts of war or violent insurrection against the United States

The Interstate Commerce Clause has been Congress’ workhorse for expanding its reach over time. When Congress wants to take an action that is not within the scope of its powers delegated by the Constitution, it often claims the right to do so by asserting that it is regulating activities that have an impact on interstate commerce.
A recent example of this was the Gun Free School Zones Act, passed in the mid-1990s, which made it a Federal crime to possess a gun within 1000 ft. of a school. A Federal District court upheld the law in response to a Constitutional challenge, saying that “is a constitutional exercise of Congress’ well defined power to regulate activities in and affecting commerce, and the `business’ of elementary, middle and high schools . . . affects interstate commerce”, a rationale that stretches the meaning of “interstate commerce” to the point where there are virtually no limits on what the Commerce Clause allows Congress to legislate. The Supreme Court rejected this broad argument and overturned the law, but Congress re-passed the act the next year, simply adding a requirement that the some kind of interstate commerce impact be proven by the prosecution.
Prof. Randy Barnett of Georgetown Law School, author of the Bill of Federalism, offers this comment on the proposed Amendment…

As Congress has exercised powers beyond those delegated to it by the Constitution, the powers of states that were reserved by the enumeration of delegated powers have been usurped. The second proposed amendment restores the Commerce Clause to its original meaning, thereby leaving wholly intrastate activities to be prohibited or regulated by the several states, or be left completely free of any regulations as states may choose. And it negates three constructions adopted by the Supreme Court to expand the reach of Congress under the Necessary and Proper Clause — sometimes called the “Sweeping Clause” — of Article I: that Congress has power to regulate wholly interstate activity that either (a) “affects” interstate activity, (b) uses instrumentalities obtained from outside the state, or (c) is part of a comprehensive national regulatory scheme. This amendment makes clear that Congress retains the power to regulate interstate pollution and the power to define and punish acts of war and insurrection against the United States, for example, the possession of weapons of mass destruction. This provision leaves untouched the delegated powers of Congress to regulate wholly intrastate activities to enforce civil rights as expressly authorized by, for example, the Thirteenth, Fourteenth, Fifteenth and Nineteenth Amendments; it only restricts the improper construction of the Commerce and Necessary and Proper Clauses to reach wholly intrastate activity.