"Arch hard" is commonly used in powerlifting circles.
Tony Bonvechio, strength coach at Cressey Sports Performance and owner of Bonvec Strength, says it's meant to create extension in the upper back, or t-spine.
This helps you avoid rounding your back and allowing your chest to cave in on Squats and Deadlifts.
No doubt, failing to extend your t-spine on these heavy lifts is an issue.
But the cue of "arch hard" is not the best cue for getting people to avoid these actions.
Yelling "arch hard" at someone mid-rep causes most people to overcompensate by extending through their lower backs. This can put your spine in a compromised position, making you more susceptible to injury.
What you really want is for the lifer to brace their core while also keeping their chest up.
"(The cue 'arch hard') is almost never used properly by coaches or interpreted properly by lifters," Bonvechio says. "Most lifters simply hyperextend their lower back, which does nothing to brace their midsection or put their spine and hips in the right position."
The Fix: "Show me the logo on your shirt and brace your core."
This will help the athlete stay in a strong position and avoid rounding their back without leaving them susceptible to hyperextension.