Using Life View Function Setting.

Life View Function Setting works after I have shoot the picture. I see the result on the back screen. I like to know how this option works on the 70D, because we have here the option to rotate the 3" screen. Handy especially with camera low at ground level for a macro shoot.
But is there also a way, for compostion purposes to see the subject on the 3" screen, before I shoot the picture?

That is live view. To see the image after taking the picture is standard image review, something which has been supported by digital cameras long before they could do live view.

Press the 'Start/Stop' button on the back of the camera

Here is page 216 of the manual. There is much more information about Live View shooting found in the pages following, give it a read.