Ok. My instructor tells me to tuck my hips forward and keep belly button back. Doing this, I have a flat back rather than a rounded or arched back.
I completely agree with your Coach. We want to ride balanced at all times, and the moment we hollow out our lower backs, we are now unbalanced as a rider - which means, if we are out - so are our horses.
Also, the moment you tuck your belly button in, and straiten your lower back - you've now activated your core - which is one of the most important factors to riding.
You will be a much more effective rider, if you sit up taller instead of leaning infront of the verticle - placing all your bodies weight onto your horses forehand. And due to that, your hands are too low.
Right now, your horse is on his forehand. If you were to sit up taller, brought your shoulders back and closed your elbow angle *lifting your hands waaayy up* you will aid your horse, lift your horse up and into your aids, getting him off of his forehand.
Again - you have a fabulous seat. I love your effectiveness - just fix the few things, and you will have a functional mount under you.