@chrisknightley , thank you very much ! actually your explanation really helped! And your english is very good 
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Now that you gave me a sense of direction, I can try and read more. I did try to look for good explanation of why the NPN was needed before the PMOS but couldn't find a decent explanation.
And small question if you will. I noticed (measured) that when the Arduino pin is LOW the NPN outputs 8.4v to the PMOS gate and the PMOS is switched off. Isn't kind of opposite ? doesn't it "spend" current that way? I mean, I do want to turn things off in this case.
Maybe it's not, i'm just asking. it probably consumes something, but very little I guess.