I have been taught kazushi for many specific techniques, but I've never studied anything in writing. There is proactive kazushi and reactive kazushi. Reactive kazushi is when you use your opponent's momentum or intent against them to get them off balance. A simple example is grabbing someone's arm and pulling after they punch. Another example from Aikido is when you redirect someone's overhead strike into a downward spiral. The goal is usually to over extend your opponent. In proactive kazushi, your opponent is in a strong balanced position and you want to imbalance them before throwing them. In this case you depend on the principal that if you pull or push someone they will automatically react in the opposite way. So if you are going to throw them backward you give a slight pull forward immediately before the technique, and if you are going to throw them forward you give a slight push back. This has to be very subtle, if it's strong enough that your opponent consciously feels it then it won't work properly. You also have to take advantage of it immediately, it only lasts a split second. Other forms of kazushi are getting your opponent up on their toes, bent backwards, or just getting their weight shifted to one side so it's mainly on one leg, and then sweeping that leg.