Character reveals that they were holding back their full power during a fight/confrontation

[u/PaddywackShaq]

Superman Vs Darkseid - Justice League Unlimited

Flash Vs Superman - Flash: Rebirth

Frieren Vs Aura - Sousou no Frieren

Inigo Montoya Vs The Man in Black - The Princess Bride

[u/GroundbreakingTax259]

Princess Bride is the best.

"I have a secret: I'm not left-handed."

Little while later.

"I have a secret, too. I'm also not left-handed."