Sifu.. i have 1 question about speedlight SB600 with D3000

When i attach the sb600 and use P-mode, the shutter will auto go to 1/60, then the flash will adjust exposure by increasing/decreasing the flash power right?

curious about how speedlight works... can some sifu explain how it works actually?

P.S: Do SB600 and Nissin 622MarkII make a good pair of flash for strobist? quite new to lightning and get both of them because of some personal reason....