The basic design consists of a frame to hold the can, with a lever and press. Simple leverage and pressure flatten the can. These designs tend to be cheap and readily available, and can even be made at home with a few pieces of wood and some screws.
More complicated designs build on the same premise with various techniques. Some can be wall mounted, some use pneumatic pressure, while some even function automatically using electric motors.
Certain models can even crush multiple cans at once, an ideal solution for someone with a lot of aluminum on their hands. This can be particularly ideal in cases where people are eating a lot of canned soups.
The thicker cans take correspondingly more force to crush, so if you can crush more of them with less effort, time and energy alike are saved.
The main catch all of all this is of course space. A crushed can takes up less than a quarter of the total space of an intact can.
For a family that goes through a lot of soda and soup but likes to recycle their cans, this can save a lot of travel back and forth to the recycling center or the sidewalk, since it will take longer to fill up the container of choice.
Even for those who don't recycle for various reasons will find cans taking up less space in the trash.
Crushers range from about ten dollars/pounds for a basic, single-can model to forty or more for multi-can or electric-driven designs.