This is Kelvin Miyahira's cue that is quoted here http://www.discussfastpitch.com/softball-hitting-technical/26554-power-triangle.html#post367373
But, how you "brush" off your rear butt cheek to start your swing is where the rubber meets the road - you should be using your inner core muscles such as the multifidus, QL's and Psoas. If you just drop your rear shoulder to laterally bend you are not using the correct muscles. It's nice to try and oversimplify a complex movement but if it was that easy everyone would be moving like the world class hitters and golfers.
Read Kelvin's article about 15 times and let the underlying biomechanics sink in.
2015-07 : How Great Players Rotate In A Golf Swing
Regarding the green line ... I see a gradient running from right to left.
See the right lateral bend in Puig and Tiger’s spine? The right elbow drives forward and you can see Puig’s shoulders tilt from the higher camera angle. Tailbone is moving away (sacrum move) from pitcher in Puig’s case and away from target in Tiger’s. That helps to create the slight arch in the lower back or lordosis.
Regarding the green line ... I see a gradient running from right to left.
See the right lateral bend in Puig and Tiger’s spine? The right elbow drives forward and you can see Puig’s shoulders tilt from the higher camera angle. Tailbone is moving away (sacrum move) from pitcher in Puig’s case and away from target in Tiger’s. That helps to create the slight arch in the lower back or lordosis.