Sit in machine with feet on platform, back under padded lever, and hips against back of seat.
Extend lower hips and low back until extended. Return and repeat.
Those with limited hip flexor flexibility may need to stop before full extension on this particular type of apparatus (fulcrum near hip). Full range of motion will vary from person to person. Many machines may not sufficiently stabilize hips or their fulcrum may be near hip's axis. In which case, synergists muscles become primary movers and erector spinae becomes stabilizer (Graves JE, et. al. 1994). See alternative back extension machines (Nautilus and MedX) with hip stabilization.
Synergists (see comments)