Synergists: Gluteus Maximus, Hamstrings, Adductor Magnus
Lie prone (on your front) on a flat bench with your torso extending off the end, and hook your heels under the bench to prevent yourself from falling forward. The end of the bench should be level with your hips to allow room for your hips to flex.
Place your hands behind your back or head, or cross them over your chest.
Inhale as you slowly lower your torso by flexing your hips and your waist.
Exhale as you raise your torso by extending your hips and your waist.
Comments and tips
To prevent the opposite end of the bench from lifting off the floor, you might need to weigh it down with a weight.
At the top of the movement, do not hyperextend your back.
The flat bench allows for limited range of motion. The ideal equipment for the hyperextension is of course the hyperextension bench. Use a flat bench only if you do not have access to a hyperextension bench.
The synergists (gluteus maximus, hamstrings, and adductor magnus) will get involved to a significant degree only if you are able to flex and extend your hips. This is not possible unless the end of the bench is level with your hips.
If you only flex and extend your waist and not your hips, not only will the synergists fail to get involved but the exercise will become an isolation exercise instead of a compound exercise.
Make the flat bench hyperextension more difficult by holding a weight plate against your chest or behind your head.