Learning how to hit a punch shot is very valuable when out on the course. Sometimes we'll be facing situations where we might have a tree in front of us or we might need to keep the ball under the wind. It can also be a safety shot when things aren't going well on the golf course.
The first part about playing a punch shot is firstly grab a longer club. You want to position the ball slightly back of middle This will help keep the ball down lower as the hands will stay more in front of the clubface through impact. It has the effect of de-lofting the club. For example a 6 iron will turn into a 4 iron when placing the ball further back.
The goal then is to swing around half way back with slight wrist hinge and then swing through impact keeping the hands in front of the clubface. The swing will feel more abbreviated on the follow through. Also at set-up you can have a little weight on the front foot. This depends on how far you want to 'punch' the ball. If you wanted to 'punch' the ball a long way, you wouldn't have as much weight on the front foot during set-up.
In situations where you might have to play a 'punch' shot off a sandy lie or thick rough, you would need to hinge your wrists more during the backswing. You would do this as you want a steeper angle of attack when striking the ball.
If you'd like any further information about how to perform a punch shot just let me know.