Besides the jab the second punch you need to know is the hook. Basically it is a lateral punch, either low or high.
The correct posture is almost a square formed with your arm if we leave out any height difference with your opponent. Your arm is held horizontally at an almost right angle in front of you and your elbow bent inwards. 110° is often described as the best angle.
Punch quick and get back into your protected position.
This is not a swing. Try not to strike out. This opens your guard widely. Besides that, your opponent sees what you are about to do with long anticipation. By the time you make the hit he has already guarded up. And lastly, though forceful stricking out uses a lot of energy which isn’t fully transmitted into the punch itself. It therefore isn’t efficient.
Hip rotation is very important with this punch. You’ll be in your regular stand, your leading foot in front. As you punch, take your body into it as well. By the end of the move your leading leg will be bend inwards.
Use the kinetic energy as your main source. This increases the power of impact significantly. It isn’t your punch itself that gives you the power, it comes from your whole body movement.
You can practice the hook aiming at your supposed opponent’s head or at his side. Hitting the chin could be a real knock-out.