In Japanese systems such as judo, jiujitsu, and aikido, a great deal of focus is directed upon breaking the balance of an opponent. This can be done different ways, but the overall theory is similar to knocking over a large fridge, it can be easily done if the appliance is resting on just one corner of its base, rather than all four.
But I think the theory extends to the mental game as well. If an opponent's thoughts can be disrupted and put off balance, the physical struggle will be minimized. Journeyman spoke about a similar topic when confronted by an attacker--acting crazy or unpredictably--thereby confusing the assailant. But the concept is integral in other situations as well, whether it be sparring, boxing, or even grappling. In this way, breaking the balance can mean changing the focus of an opponent (feint, softening strike, disceptive telegraphing); it could mean intimidation or confidence; it could mean drawing the opponent into a vulnerable position by showing him/her an opening in your guard.
In judo, for example, it is common to push forward as if looking for a throw, and when the opponent resists and pushes back in response to this force, landing the sought after move through a pulling motion instead.
This becomes a merging of mental and physical balance breaking.
And momentum is crucial to both.