pf2e.fyi
From Pathfinder #159: All or Nothing, which isn't enabled.

Ambushing Knockdown

FEAT 8
UNCOMMONROGUE
Prerequisites Ruffian Rogue
Access Side with the Washboard Dogs in the gang conflict.
Requirements You're wielding a melee weapon that deals bludgeoning damage and you're Hidden from or Undetected by an enemy.

You emerge from hiding and deliver a blow that can knock an enemy sprawling to the ground. You Stride up to half your Speed, but you must end your movement next to a foe you're hidden from or undetected by. You then Strike that enemy with the required weapon; if you hit, the target is knocked Prone.

Pathfinder #159: All or NothingOGL