One of the most important aspects of controlling Uke's balance is twisting Uke's body and controlling the shoulders. This is frequently done by controlling the elbows. Chine In, in Spring Forest Qigong, says that we direct the Ki by directing the elbows. He is talking about directing your own Ki for health. Ki is not the issue here but the use of your elbows and the control of Uke's elbows is one of the most important aspects of Aikido. When Americans drink from a cup or a glass they hold the elbow to the side and raise the cup by raising the elbow at the side of the body. From a martial arts point this is dangerous. A raised elbow like that is easy to throw with Ikkyo. If someone hits the cup the content goes into the face of the person holding the cup. The Aikido way is to hold the elbow against the body and have the hand around the cup near the top. Now the elbow is not available for attack and if the cup is struck from the front the natural mechanics will make it tend to turn back toward the attacker. In Aikido the elbows are kept down except when rolling or completing certain throws. This is an important part of sword raising or furi kaburu. With the elbow down and to the front and the elbow fully bent the open hand is in front of the face. A very slight lift of the elbow by moving the shoulder brings the hand to the forehead. This is a very powerful and easy to raise the hand without straining the shoulder.
If Uke grasp nage's wrist nage's elbow almost always moves toward Uke's elbow. This seems easy but it is not. It is a fundamental principle of Aikido to not push and pull Uke at the same time. When Uke grabs Nage's arm with two hands and Nage tries to move in Uke will hold Nage out. If Nage resists this Nage will push one hand and pull the other. This is not good. It is better for Nage's hips to drop so that Nage's head can move to Nage's hands. Now Nage's arm can rotate in Uke's hands turning Uke and allowing Nage to turn. Once this is done Nage can easily move Uke and Nage's elbow can easily move to Uke's elbow. Moving the elbow moves the shoulder and twist the body disturbing Uke's balance.
This principle should be applied where ever possible. Whether Nage wants Uke's elbow to move up above Uke's hand or to move down below Uke's hand the easiest way is for Nage's elbow to move to Uke's elbow. This makes the correct path for Nage to press a continuous attack keeping Uke off balance and defensive.