If your gym has these, the principle of use is the same. Toes forward, try not to arch your back. Let's see what you're made of when you give these a shot. Manual hamstring curls are an old-style strongman and woman exercise that is deceptively simple. The first time I tried doing even a controlled negative of these, I fell flat on my face.
Most people begin by using a counterweight, as shown above.
I warn you, they look goofy. Then, keeping the body straight and rigid, and bending only at the knee, one raises and lowers the body, using the hamstrings to curl upright again. I'm demonstrating this exercise with the counterweight on a lat pulldown machine. It's an easy one to use, since I can just slide my lower legs underneath the knee pad, set at its lowest setting. In my gym, the knee pad doesn't go down as far as it should, so I have to bend my legs a bit more than is correct.