Besides communication, the rod (or, physical discipline) is the second key element in training children. It is important to remember that these two methods are designed to work together (although which is used more does change depending on the age and maturity of the child).
Before we discuss what "the rod" is, we must understand the nature of the problem that we are trying to address. The Bible teaches that a person does wrong not because of a lack of information in his head, but because of wickedness (Jeremiah 17:9) and foolishness (Proverbs 22:15) in his heart. A child is wicked because he is primarily concerned for his own wants, and he is foolish because his concern is for his immediate wants rather than God's ultimate best.
Discipline is not about being angry at the child, but about teaching the child to obey and about rescuing her from the spiritual death that comes from rebellion. For a child to refuse to submit to his parents' God-given authority is foolish behavior, as she is rejecting God. A spanking, given in a Biblical manner (more on this later in the book), helps prepare a rebellious heart "to receive life-giving words."
Physical discipline is a parental exercise that shows your faith in and obedience to God, and it's "an expression of love and commitment" to your child (see Hebrews 12). Think of it as a rescue mission: my rebellious child has distanced himself from god, and I must be willing to do anything to reach his heart with the Gospel. God has given us a wonderful instrument to help do this -- the rod.
Tripp gives a good list of common distortions and objections to the rod, and I invited you to prayerfully consider that list to see where you have been misled or mistaken. For me, I must remember that the rod is not about retribution (my child paying for his sins); it is a teaching tool, not for punitive reasons.
How have you used this element of child training? What questions have you had?