Back extensions strengthen the lower back and hamstrings. If the lower back is inhibiting the progress on squat or deadlift, this is a good supplementary exercise.
This can be done in multiple ways depending on the equipment available. The basis for the exercise remains the same throughout.
The variations in back extensions are almost identical in concept, however, they allow the user to work the muscles through different movements.
Lie face-down on the ground. Bring your legs together and place your arms out front. Lift the chest and legs off the ground. Hold the position anywhere from 10-60 seconds.
Sit in the resting position provided by the machine. Set the weight to what is comfortable. Keep a straight back, then lean backwards.