I'm currently reading and studying A.S. Medvedyev's "A System of Multi-Year Training in Weightlifting" and "A Program of Multi-Year Training in Weightlifting." Medvedyev is undoubtedly one of the top Sports Scientists in history and is one of a handful of Russian scientists/coaches who literally wrote the book when it comes to weightlifting training.
Here's an excerpt from the "System" book that I find very relevant:
"Phase IV -- the Final Acceleration… The posture at the end of the phase is as follows: the legs are completely straight; THE TRAPEZIUS MUSCLES ARE ACTIVELY WORKING; THE ELBOWS ARE FLEXED; THE ATHLETE IS STANDING ON THE TOES and is ready to execute the squat-under.
The Objective of Phase IV is to achieve maximal barbell velocity at THE GREATEST HEIGHT POSSIBLE, utilizing the power of the legs and the torso…with the subsequent maximum amplitude of movement in the JOINTS OF THE LOWER EXTREMITIES." (pp. 55)
(To note, I've added the caps and underlines for emphasis)
We also have had the pleasure of hosting and spending time with high level Russian and European lifters during their seminars over the past 8 weeks. They perform and teach the lifts just as Medvedyev wrote about almost 25 years ago.
To me, there is no debate on what the traps, arms, and lower legs should be doing when lifting: raising the height of the bar. While it may not look like that happens with heavier weights, it's the INTENT to shrug and pull the bar as high as we can up on our toes.