Power stancing was first introduced to the Souls series in Dark Souls 2, and though that game has the worst reputation of any in the series, the mechanic was well liked enough to return in FromSoftware's latest opus, Elden Ring. Here's what you need to know to power stance the weapon of your choosing.
Elden Ring Power Stance Guide
Power stance is the fancy name FromSoftware gave to wielding two weapons of the same type — one in each hand. You can see a weapon's type just under its name in the Equipment screen. As long as their weapon types match, you should be able to power stance the two weapons in question. (The Wakizashi is one exception — it power stances with katanas rather than other daggers.)
Once you've equipped the same weapon type in each hand, you'll be able to access the power stance move set by using the same button you would normally use to block. On controllers, that means the left bumper. This move set deals unparalleled damage, as it allows you to hit enemies with both weapons at once. You still retain access to your normal attacks with the right triggers, and to your Ash of War with the left trigger.
There are a few drawbacks. The most obvious is that power stancing makes it impossible to block. Wielding two weapons at once can also be quite heavy, especially if you're power stancing something like a greatsword. With a little investment into Endurance, you shouldn't run into too much trouble.