With the release of Super Smash Bros. Ultimate — and the recovery frames added to exiting shield — powershielding has become less pragmatic. But the newly-found parry mechanic out of shield has gained massive prominence thanks to the frame advantage — similar to Street Fighter III‘s parry system — given to you if you execute it.
With the input window of the parry being tight, ZeRo explains exactly how the system works, and even gives pointers on how to practice it before taking it up against real opponents. He also explains that it is not necessarily a mechanic you want to abuse on every single move, but will help you take advantage of the depth of the Ultimate system.