Bracing for a Lift

Don’t really know how to describe this, but, I will try.
When setting up for a heavy lift, be it a single or whatever. I tighten up the whole body. When executing the lift, I maintain my tightness, right up to the top of my head.
Something I noticed for many years but never really realized til today was something interesting.
If i release the tension from my neck - up,(in other words, basically just keep tight everything from the traps down)–
I can 1. perform more reps, and 2. do more weight.

Have you ever encountered something like this?
If so, what is it attributed to?