Comparing squat variations, Schwanbeck et al. (2009) explored lower erector spinae muscle activity in the back squat with either a barbell or in a smith machine with the same relative loads (8RM). (strengthandconditioningresearch.com)
It's performed almost exactly like a normal back extension... the lateral pulling tension is going to really hit those small stabilizer muscles of the lower back and spine and help you develop much better spinal strength and stability, while also working the bigger spinal erector muscles. (fitstep.com)
The prone trunk extension (superman) exercise is an effective exercise and displays very high levels of erector spinae muscle activity when performed with maximum lumbar extension. (strengthandconditioningresearch.com)