Basically, you just perform a regular squat. however, at the botom you pause on a box or a bench that os positioned slightly below parallel. After the pause you squat up, as quickly and as explosively as possible. Definately a great movement to develop leg strength.
smith machine is a no no. (for everything...but box squats evenmore so)
i find box squats help target different muscles than regular squats.
like my glutes and hamstrings more than the quads.
it has really help the first 3 or 4 inchs from the bottom of the lift
It will improve your squat.....i dont believe it to be a total substitute.
it depends what your goals are and what you train for.