Caesar's elite cadre of bodyguards, the legendary praetorians, use an aggressive Legion Assault to brutalize enemies. Perform a Power Attack while running forward to execute the Legion Assault.”— In-game description.
It is acquired by talking to Lucius in Caesar's Tent with an Unarmed skill of 50, and if your reputation with Caesar's Legion is "Accepted" or above. Since it is a dialogue check, skill magazines and chems like Buffout can help you meet the requirements.
Perform a power attack while running forward to perform the Legion Assault. When performing this move in third person, the Courier runs forward, leaps into the air, and thrusts their right palm forward before landing back on the ground.