When does the camera use the mechanical shutter and when does it use the electronic shutter?

I notice that Nikon P510 is having a 'Electronically controlled mechanical and CMOS electronic' shutter. So, I was wondering when will the camera use the mechanical shutter and when will it use the electronic shutter?

From what I know, mechanical shutter is like a candle. The more you use, the candle life become shorter and shorter until it die. However, this is not the case for a electronic shutter. (Please do correct me if I am wrong.)