One leg at a time so neither side hides.
Don't swing or arch, let the working hamstring do all the work.
Curling one leg at a time on the machine means each hamstring moves its own load through the full arc from straight to heel-at-glute, with no help from a stronger partner leg. The pad's fixed path keeps the work isolated, so all you have to manage is tempo and honesty. If your two-leg curls have been carried by one side for months, this is where you find out.
Two-leg curls let a dominant hamstring do more than its share without you noticing. One leg at a time forces each side to match reps and load, which evens out imbalances that show up later in sprinting and heavy hinging.
The weaker one, while you are fresh, then match its reps with the stronger leg. That caps the strong side and gives the lagging hamstring the best quality work.
The weight is too heavy and your body is arching for leverage. Keep your hips pinned down, lighten the stack, and make the working hamstring pull the pad through the whole range on its own.
