It is obvious, even to someone who has never studied Judo, that a throw will be more effective and require less strength when applied to an opponent who is in a weakened state of balance. One of the things that distinguishes a beginner from a more advanced student, is the ability to focus attention on your opponent's balance. It is one of the differences between learning a throw, and being able to effectively apply a throw. Many martial artists tell me that they know how to do basic Judo throws like seoinage or osotogari for example. However, learning to get your body into the proper position to effectively apply your strength is only part of learning a Judo throw.
Most people do a fine job of getting off-balance by themselves.
One way to apply Judo throws is simply to catch the opponent at every opportunity that he or she gives you. It is just not possible to move without some degree of imbalance. It is certainly not easy for your opponent to attack you without a force and movement that makes him or her vulnerable to your counter attack. However learning to take advantage of your opponent's imbalance requires a finely tuned a sense of timing. In order to be at the right place at the right time, you generally have to anticipate the opponent's movements, a skilled that requires a great deal of experience.