Does holding your gun diagonally like John wick have practical benefits?

I have no idea who John Wick is, but a slight rotation of the pistol of up to to about 20 degrees from vertical is commonly referred to as the “McMillan Tilt”. It was developed by pistol trainer and world champion competitor Bill McMillan, LtC , USMC.

McMillan knew what he was doing- he won the Olympic Gold Medal in 1960 in Rapid Fire 25 meter Pistol. Other major victories include one World Championship and five Pan Am Games Gold Medals, plus numerous silver medals. He found that the technique allowed him to fire with accuracy at greater speed.

The McMillan tilt technique allows for the bones in the arm to line up in direct support against the recoil of the gun and is a significant aid in controlling a pistol in Single handed rapid fire, allowing it to recoil directly back against the resisting bone structure And thus pop back down onto target more rapidly.

The McMillan Tilt is NOT the Sideways “Gangsta” grip, which makes the sights irrelevant and causes the weapon to recoil sideways to the left for a right-handed shooter and producing significant misses. Of course, the “Gangsta” looks “cool”.

The sideways hold apparently appeared after a writer/ producer for the once-popular TV show “Jake and the Fat Man” saw the Tilt being taught at a shooting school and decided that it would make a good “signature” for one of the main characters. Like all things in Hollywood, if some is good, more must be better, and the character “ Jake” always held his pistol 90 degrees sideways. It has been a fixture of street culture ever since. I'm OK with it- it makes bad guys miss.

The McMillan Tilt in modern usage is taught by such trainers as Massad Ayoob of the Massad Ayoob Group. It can be a useful technique for some people in close range, rapid fire single hand engagements with full size pistols. Properly performed, many shooters find it helpful, but like many techniques, its utility varies from shooter to shooter. It is clearly not for use with service pistols past about 5 yards, at which point accuracy begins to degrade, but at close range with full power ammunition it can be a helpful technique.