if you mean so the engine is shown thru the bonnet ie the bonnet is see thru this is how i do it
this is my first tut so bear with me
1:- you need 2 images of the front of the car one with the bonnet closed

and a second identical one but with the bonnet open
2:- now you can do either of 2 things you can use youre polygonal tool and select the area of youre engine you want to show or as i have and use the elliptical tool
3:- was you have selected youre area you need to then feather it (i set it to 10 pixles which i found just right)

then copy that selection
4:-now go to youre image with the bonnet closed and paste the engine onto that
5:- looks like the engine has took a bit of a slide so the next thing to do is to is go to edit free transform

and move the engine to where it should be
6:- after you have positioned the engine where you want it just merge the layers and there you have it 1 see thru bonnet

this is just a quick but im sure you can get better results by playing around