Need help understanding turbo surge from lack of preasure (air flow maps help)

OK so i've been learning about turbos and turbo air flow maps (to help me understand about turbo) and i understand turbos can get compression surge from getting to much back pressure say without a wastegate or what ever, and thats pretty easy to understand as the pressure build up can slow down the turbine.

And now I learnt that turbos can get surge from lack of pressure from what i think i've learnt, according to these air flow maps? or am i reading them wrong?

http://www.automotivearticles.com/uploads/flow_map.jpg

So according to the flow maps if you are trying to boost too much with little air flow, then you get surge? which i guess happens more on big turbos on small engines like lets use this one for example

http://www.turbobygarrett.com/turbobygarrett/images/catalog/Turbochargers/gt60_images/731377-1comp_t.jpg

^^^^this turbo on a small displacement engine.

I know that most times if its only a bit off to the left of the surge area you get lag, but I've told (or i've read) this is bad for the turbo if your too much into the surge area?

whys that? and how does it damage the turbo from lack of pressure?

Thanks