Kuzushi is a Japanese term for unbalancing an opponent.
The noun comes from the transitive verb kuzusu, meaning to level, pull down, destroy or demolish. As such, it refers to not just an unbalancing, but the process of putting an opponent to a position, where stability, and hence the ability to regain uncompromised balance for attacking, is destroyed.
In judo, it is considered an essential principle and the first of three stages to a successful throwing technique: kuzushi, tsukuri (fitting or entering) and kake (execution).
The methods of effecting kuzushi depend on maai (combative distance) and other circumstances. It can be achieved using tai sabaki (body positioning and weak lines), taking advantage of the opponents actions (push when pulled, pull when pushed), atemi (strikes), or a combination of all three.
Kuzushi Technique by Neil Adams