The nearer an object's centre of gravity is to the middle of the base of support, the more stable the object. Whereas the closer it comes to the margin of the base of support, the less stable it is. If an object's centre of gravity tilts past the edge of the base of support, the object falls over. For this reason, carrying a suitcase in one hand is destabilizing, as it tilts one's centre of gravity toward one side of the supporting base (in this case, one's legs), closer to the bounds of the base. One way to maintain balance when tilted to one side is through compensation, the intentional leaning in the opposite direction. Thus one carrying a suitcase will typically lean toward the opposite side in order to keep his centre of gravity within the bounds of the base of support.
Another aspect crucial to an object's stability is the height of its centre of gravity. Objects with a lower centre of gravity are more stable than those that are 'top heavy'. If the centre of gravity is very low, an object may even need to be lifted for it to be tipped over. For example, in order to tip over a couch, one actually has to lift its centre of gravity first, for a mere push will only move the couch to another location. In order to tip over a lamp, however, one need only give it a push. Olympic wrestlers know this principle well and use it to their advantage. They keep their centre of gravity as low to the ground as possible in order to make it difficult for their opponent to throw them over and pin them down.
In skateboarding these principles are pervasive. Almost every trick involves the implementation of these principles in some way. Here are a few simple examples:
1. When landing from a high drop the board sometimes 'freezes', sending the skater flying forward. Compensating, by leaning back a little in anticipation of a possible 'freeze' can help one maintain or regain balance.
2. When ollying into a bank or a ramp where one often tips back on landing, it's a good idea to compensate by leaning forward a bit more than usual. (In both cases, only practice allows one to gauge precisely how much compensation is required.)
3. When landing from a height, such as the completion of an Ollie over a set of stairs (especially when landing in an awkward stance such as fakie, and all the more so switch) , crouching down on landing lowers one's centre of gravity and can dramatically improve stability.
4. When performing a manual, where one rides only on the back two wheels, the skater usually oscillates between his front and back feet/shoulders in order to balance - a continuous process of compensation. However, adept skaters can manual without any visible scaling. They've developed the skill of finding near perfect equilibrium between the counterbalancing forces acting on the board: the force applied by their back foot/shoulder on the board's tail, and the force of their front foot/shoulder on the top set of bolts. This type of graceful balance is referred to as static equilibrium, and the oft heard exclamation, 'that's smooth!', well describes such style...