I've found the recommendation (traps into bar, hips forward) very helpful.Key Points:
1) The origins and insertions of the hamstrings and rectus femoris allow them to extend the hip and knee simultaneously, even though their actions oppose each other.
2) Two joint muscles allow force from single joint muscles to be transmitted to joints they wouldn’t otherwise be able to effect. For example, the rectus femoris allows the glutes to help extend the knee.
3) You can put these principles to work for you by learning the best way to grind through your sticking point on a squat
4) Because two joint muscles transmit force throughout all of your hip and thigh musculature, squats aren’t truly knee or hip dominant, regardless of how they look or what the external torques at the joints are.
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By driving your traps back into the bar (to provoke a stronger contraction from your hamstrings, similar to a good morning), and squeezing your glutes to drive your knees forward and out (transferred down your rectus femoris, resulting in increased knee extension torque to keep your knees from flexing as more stress is shifted to your quads), you shift your hips forward slightly. That shortens the moment arm they’re working again, and voila! Same weight + shorter moment arm = less hip extension torque necessary to get through your sticking point and finish the movement.
https://www.strongerbyscience.com/squat ... ominant-3/
