Traps start near the spine at mid- to low-back and go up forming a 'V' to the shoulders, etc. Also, typically, it's the downward pull on the traps that cause more growth than the actual upward and/or pause at the top.
For years, I did heavy shrugs, light shrugs, etc. pausing at the top with little to no success. Once I started doing deadlifts, my traps popped right out.
I make sure I do deadlifts once a week and supplement deadlifts with shrugs on a different day. Dumbbell shrugs, to me, are a waste, not heavy enough Dumbbells to really do much, unless I want to be doing 50 reps. The Smith machine forces you to move your body to accommodate the machine, instead of the weight moving to accommodate your body (bad idea, I think).
If you don't do deadlifts, you should. If you do already. you're probably thinking that it's the shrugs that are making your trap muscles pop out.....or maybe you're one of the lucky ones whos traps respond well (mine don't).