How come {=SUM(A2:A10*B2:B10)} works, but without CTRL+SHIFT+ENTER it doesn't?

I'm trying to learn more about array formulas, but I wish there was some guide about what goes on behind-the-scenes. Doesn't A2:A10*B2:B10 simply spit out an array of multiplications? In which case, SUM should be able to just consume that? But here we're using the Array Formula syntax, which means we are using the AF version of SUM? Does that mean it iterates through each multiplication, and SUM them up, and stitch them together somehow? Really confused.

`(A2*B2)+(A3*B3)+....`

– Scott Craner Jun 16 '16 at 21:22