What it works: Your legs, especially your quads. It works more muscle groups than a leg extension, but fewer than a barbell squat.
How to use it: Here’s where it gets tricky: there are many different types of leg press. Some use a selectorized stack with a pin, while others use the same plates that the barbells use. In some machines, the footplate moves, while in others, your seat slides on a track, or both. Hack squat machines are similar in function, but you’re standing up while supporting the weight via pads on your shoulders.
Important adjustments: Make sure the starting position allows your legs to be slightly bent. On the kind where there’s a weight sled above you, you’ll straighten your legs to lift the sled, then release a lever, and then the machine will be able to slide enough to let you fully bend your knees. If a tall person used the machine before you, don’t try to get in there and release it by getting onto your tiptoes; either empty the machine and reset it, or ask that tall person if they wouldn’t mind helping you set it to a shorter height.