Machines that operate with cables attatched to the movement arm tend to
distribute the weight more evenly through the movement and for that reason I
recommend them before simpler, lever-based implementations.
You won't be able to do as much weight on this as you do on leg extensions.
If you've never performed this movement before, use about one third of the
weight you use on leg extensions.
Lie face down on the machine and hook your ankles behind the pad. Grab the
handles below you to give you more leverage in the pulling motion. Your knees
should be just a tad beyond the edge of the bench so that you can freely and
comfortably bend them.
Curl your legs up until the pad touches your butt--or gets as close to it
as is reasonably possible. Lower it back down slowly. Barely allow the stack
to touch the bottom before raising it back again.
This is my preferred hamstring isolation movement.