This answer is in reply to @Dave Liepmann's query, and is in support of Trevoke's answer. No need to upvote this one.
Dave Liepmann asked, "So, unbalancing and locking, or unbalancing or locking?"
This is a common way to frame this concept. When your body has not learned this stuff, your mind wants to put this into neat boxes because the underlying principle is still too abstract.
This isn't the full explanation, but it will do for now: Every human being has structure underlying their bodies and mind. The structure that underly the human body is the skeleton; the structure that underly the human mind is the ego (learned self). Disruption of the structure is what allows a weaker person to defeat a stronger person: you don't fell a mighty tree by toppling it from the top.
Uprooting is one tactic by which someone disrupts structure (by undermining the person's power base). Locking is one tactic by which someone disrupts structure (through unexpected and/or painful manipulation of structure). They are different, yet they are the same.
Disrupting "balance" is an many-layered inside-joke whose first layer of meaning refers to "kicking someone out of their comfort zone." Falling is one of the two basic fear instincts wired into the human body since birth. When we feel we are in free-fall (aka, "unbalanced"), we tend to instinctively and immediately try to stop falling. For the untrained, this instinct is powerful enough to override higher-brain function. By undermining or manipulating someone's structure in a way to trigger this instinct, that person will be so busy trying to right himself that he will not notice what you are really doing.
And some art derive their entire art from falling (and rolling out of falls).
This is also why, one way or another, all combat-effective traditional arts spend so much time on body structure. (Why would you give your enemy a broken structure? Well ... ) You learn the right way to carry yourself; you learn how to disrupt structure by experiencing your weakness.
This goes much, much deeper. (For example, why did Cheng Man-ch'ing say, "Invest in loss?") If you want the full explanation, you'll need understand what Sun Tzu, Musashi, and Col. John Richard Boyd were saying in common.