The trapezius is the huge muscle on your upper back. Contrary to popular belief, it is not just targeted with shrugs. Its main functions are scapular elevation, retraction, and depression as well as neck extension. To hit all these functions, and thus all the heads of the traps, you have to carefully design your program as outlined. If you do shrugs and don't feel your traps, that may be because your levator scapulae. To target the traps, slightly bend over and have a wide grip.
If you enjoy this weird little reference you may also be interested in the updated, revamped PDF version: A Field Guide to Flexbox. While I am no stranger to the magic of Flexbox, I find that I am not using it very often just yet. As a result I end