Why? - simple physics.  As Taekwondo students, we are constantly training and strengthening the core muscles around our hips, lower back, and abdomen.  As we kick and punch, the contraction of these muscles transfers the power developed through our core's rotation to the part of our bodies being used to strike the target.  Imagine kicking a target using only your leg, while keeping your arms hanging loosely at your side.  Your left foot is in a straight ahead position and you attempt to kick with your right foot.  In most cases, the muscles in your hip joint will not be strong enough to lift your leg up parallel to the floor, let alone make any forceful contact with the target.

Many new students compensate for this fact by getting a running start on virtually every movement (you know who you are, and so do we).  Over time we learn that by pivoting our plant foot, we add rotation to the movement of our right leg.  This movement adds strength to the right leg - in essence your right leg is now borrowing strength from your left leg.  In order for this transfer to occur however, we have to connect the two legs together through the hips, lower back, and abdomen.  As we tighten these muscle groups, the amount of velocity produced in the right leg increases substantially.  So tensing your core muscles when kicking or striking is like tightening up your belt to connect your pants to your body.  Now think of a kihap as inhaling and taking your belt in one more notch.  Your pants are now securely attached to you, and in the same way the kihap helps us attach the power of the hips and legs together by forcing our abdomen to contract that extra little bit to maximize power and speed.

Where? - an effective kihap makes use of the diaphragm.  Society has trained us to suck in our guts to appear thinner.  Unfortunately, this prevents us from breathing properly.  If you've ever watched a baby sleeping, you know that we naturally move our stomachs in and out making use of the diaphragm.  By focusing on diaphragm contraction, we can all breathe better, speak with more intonation, and kihap louder!

When? - should be timed so that your kihap and contact with the target occur at the exact same time.

Who? - everyone.  This is important during sparring matches as well.  Let's say you and your opponent kick each other at the same time, and in the process your opponent loses his/her balance.  If your kihap is loud, you have a better chance of being awarded the point because the sound made is perceived as superiority.  In other words, the judges will assume the force of your kick made your opponent stumble and the kihap focuses the attention on you.  So in this case 1+1 = 1point.

 The sound we make when we kihap is as personal as what clothes you wear.  You can start by just saying the word kihap! or check out YouTube to see what other people do.  What sound you make isn't important (within reason of course).  What is important is developing the good habit of doing it with each punch or kick to focus your energy.