Why are we leaning back?

If you watch videos of Ip Mann or Si Gong doing one of the forms it looks like they are leaning back. Someone asked a out this in class today. It is an illusion. Yes thier torso may be angled backwards however the whole time they never compromise their balance. Their centre of mass is still directly over their feet. 

As sifu always says, you should never compromise your balance or your spine. 

So what's the point of angling your torso like this?

Sifu says that they are relaxing their back muscles by rotating their shoulders backwards so that their chests can relax. They also rotate their hips the same way to balance this. 

I still don't quite understand how to do this properly but the the question does show a fixation on the external. How something looks on the outside. Rather than looking at what underlies it.