This cue has good intentions, but for a novice lifter, it can be easily misinterpreted.
Many of the same issues with the cue of "arch hard" are present with the cue of "push your abs out".
Namely, the lifter may interpret the cue as they're needing to hyperextend through the lower back.
This makes them both weaker and more susceptible to injury.
"It is physiologically impossible to brace musculature in a contracted state and actually push out at the same time," asserts Dr. John Rusin, a strength and conditioning coach who specializes in sports performance physical therapy and rehabilitation.
Our first three bad pieces of workout advice all deal with the concept of bracing your core, as it's an incredibly important aspect of training with significant loads.
Again, a better cue would be "Pretend I'm about to punch you in the stomach. What would you do?".
It's an almost primal instinct to brace your core when preparing for an impact.
The coach, teammate or lifting partner can even poke the lifter's core as they stand up with the bar to make sure their core is rigid.
The Fix: "Brace your core as if you are about to get punched in the stomach."