Primary muscles worked are the hamstrings.

Raise the seat to horizontal position. Adjust the height of the bench unit so your knee is just under the seat. Position your self with your supporting leg on the side of the bench unit. Adjust the leg pad so it is resting comfortably on the back of your ankle. Curl the weight up until you reach full contraction in your hamstrings. Don’t put on so much weight so you limit your range of motion. Slowly and under control lower the weight to the start position. Repeat with your other leg when you have finished your desired amount of reps.

You can adjust the start position by changing the angle on the bench unit.