the ribs thing is part of the "winding" I
couldn't find any videos of master chen doing this, I don't think that he's so into putting things on youtube etc...but, I found this, which looks very similar, drop ribs/chest, opening while winding up under his left armpit & then releases the wind towards the end of the forward movement.
http://www.youtube.com/watch?v=t5OgzOXy38U