A punch shot is one that is played with the intent of lowering the golf ball's trajectory in flight through a couple changes to a golfer's normal stance and swing. Why would you want to hit the ball lower? The most common reasons are:
To lessen the effects of a strong wind on the golf ball in flight. If you are hitting into a headwind or strong crosswind, the golf ball will be more affected the higher it flies. So the golfer might choose to play a punch shot (to lower the trajectory of the ball flight) to negate some of the effect of that wind.
Another reason to intentionally lower the ball flight with a punch shot is because you need to get the ball underneath something that is in your normal line of flight. Most often this means the golfer is trying to keep a golf ball low to get underneath overhanging tree branches.
Some golfers who master a punch shot also find that it gives them better control and accuracy, and so sometimes might choose to play a punch for that reason. Tiger Woods, for example, is well-known for playing the shot he calls a "stinger" (basically, what the rest of us call a punch shot) from the tee on tight driving holes.
So how do you play a punch shot?
1. Take more club than you would normally use from the distance you are playing. If you are 150 yards out and would typically use 7-iron, take a 6-iron or even 5-iron for a punch shot. (Take into account the strength of the wind and also how well your practice of punch shots has gone. Because you're going to practice this shot, right? You need to, if you want to pull it off on the golf course when you need the shot.)
2. Set up with the golf ball farther back in your stance than you do for one of your normal swings. The middle of your stance should be the most forward you'd play a ball on a punch shot.
3. Make sure your hands are ahead of the golf ball at address (and at impact); use a forward press or move the ball back farther in your stance if necessary.
4. Put about 60-percent of your weight on your front foot.
5. Now make a smooth but abbreviated swing with limited weight shift (think of swinging mostly with your arms) — take the club back about three-quarters of your normal backswing. Swing smoothly, accelerating through impact, and cut your follow-through short, to about three-quarters of normal.
The combination of these stance and swing adjustments should produce a lower ball flight that better cuts through the wind and better holds its line in the wind or that helps you get under those tree branches.
Keep in mind that a low shot will roll out more once the ball hits the ground. This factor makes punch shots an option on firm-and-fast golf courses, and for hitting into firm greens that are not protected in front by any hazards. Punch shots are options for golfers who want to roll the ball up to such a green.