"Close the loop" like a triangle by attaching your hand to your chest. When your upper body moves the hand should remain largely attached so that the movement you create transfers through your overhook into your opponents body.
An overhook can pressure them up or down. To pressure up, stand on your toes and lean laterally (not forward). This takes the "slack" out of their shoulder / scapula so that when you go for your throw you are fully connected. (This operates on the same principle as taking the slack out of a Gi).
When you take an underhook or overhook, use your freehand move their head so you can take dominant head position. "Wax off" to move their head and replace it with yours, pressing your forehead into their temple.