Neutral spine training ensures the optimal instantaneous axis of rotation around the spinal joints for better posture, function, abdominal tone, resistance to injury, strength and performance. Optimal pull from muscles in ‘balance’ with each other results in no disruption of the instantaneous axis of rotation and therefore no significant trauma to the joint.
Neutral spine philosophy is is not based on the idea that the spine should always be in neutral, but that the neutral position should be the strongest position that the body moves around equally. The body will always migrate to its position of strength and so always migrating or coming back to neutral is ideal.
If, when measured, your thoracic and lumbar curve in not around 30-35 degrees, I recommend corrective exercises to help neutralise the spinal curves. I also recommend functional training with a neutral spine up to 60% intensity (6 out of 10 on a rating of perceived exertion scale). As a guidance 60% of 1 repetition maximum equates to a weight you could lift 15-16 times (reps) and so you might ‘neutral spine’ train with this weight for 8-10 reps.
Once ‘neutral spine’ is your strongest position, that you constantly migrate back to, you can then move onto heavy strength training, power training and power/endurance training, performing them better and with a reduced chance of injury.