When they drop their hands

One of the most important lessons in boxing or kickboxing is that if a fighter drops his hands or lowers them by accident, a free line is opened so that an attack can be launched. If nothing is protecting your face, then it can be hit easily by an oncoming punch.

This is a defensive explanation to the subject, but this it can also be taken into consideration from an offensive perspective as well. That is, you can con your opponent into dropping his hands.

For example, if you repeatedly fire body shots that force an opponent to repeatedly use an elbow cover, then the opponent may lower his arms so much that a head shot opening has been presented. So, a body shot to head shot combo will have devastating effects.