In Theory Edit
The Unbalanced Defense is a defensive strategy in which the player sacrifices certain areas/cities in order to strengthen their defense in other areas. This type of defense works well for most regions (except Europe) and works better for some than others. The loss of the undefended cities is balanced by the decreased chance of the defended cities being destroyed.
In Practice Edit
A player in Russia may sacrifice all of Eastern Russia (Except for the North-East coast, placing one there allows easy access to North America). And concentrate those silos to strengthen the Moscow area, allowing easy strikes to Europe, Africa, and possibly South Asia. This allows greater defense of 95% of their population instead of spreading too thin in such a large territory.
South Asia Edit
A player using an Unbalanced Defense in South Asia may leave Tehran, and possibly India, undefended (no Missile Silos nearby), counting those cities as acceptable/inevitable losses. This will leave the player with two to three extra missile silos to place on the eastern side of South Asia, concentrating their anti-Missile defenses around the major population centers (Tokyo, Seoul, Beijing, etc).
In Africa you might want to ignore Cairo, even though it is your largest city, as it is very close to several other continents and instead focus on the central and southern parts of Africa. Those cities are closer to each other and a lot easier to defend, and with a big empty space in between you and the northern continents you will have much more space for shooting down incoming nukes.
North America Edit
Some players commanding North America prefer to abandon the west coast, focusing defence on the Eastern Seaboard. However, Unbalanced Defense may not be as effective for North America as there is more significant population on the West Coast.