If you are just browsing I recommend that you read the precursory articles to this one; The Jab and The Straight Right
This is a punch that in my view is one of the most effective at ending fights and there are 4 basic reasons why this punch is so effectual;
When you're talking hooks whether it be left hook or right hook you're talking about torque. That is turning power, think electrical drills, wheels, engines etc.
Torque, also called moment or moment of force, is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist.
When executing the left hook your body is acting as the axis about which your left arm will rotate with torque.
So as always you'll be in your orthodox boxing stance and the best way to learn this punch is to have someone with punch pads helping you out. For the sakes of this article I'll make do with an imaginary white line. You could also hang a rope from the ceiling just to learn and practice the basic movement before you go out hitting stuff.
To set this punch up, the target should be just off centre as shown below with the white line.
To get your full body weight behind this punch requires that you 'turn the toe'. As you can see in the pictures, my foot has gone from pointing straight ahead to about 90 degrees to my right. And that is how you get your body weight behind the left hook. And as always your other hand is guarding the chin
Now, when you follow through you are going to be in an excellent position to launch a vicious straight right and often when you do your pad work with a trainer those are the sorts of things you will be learning. It's called chaining your punches together and is an extremely efficient way to use your bodyweight, momentum, rhythm, timing and energy levels. One thing flows from another.
Also notice how my left arm is parallel to the ground, that's important when learning this punch so try to keep it on that level.
In the act of turning the toe, you are actually turning your feet, legs, hips and upper torso to create that power you need.
You'll notice that all punches start from the ground and work their way up - that's why if your footwork is stuffed up you've got no power.
Imagine someone trying to punch while floating in space, there would be absolutely no force behind it whatsoever.