Activate Shout: When entering a new area or when your buff expires, press your chosen Shout button Diablo 4 Items. This provides significant offensive and/or defensive buffs, enhancing your effectiveness.

Whirl through Enemies: Hold down the Whirlwind button and move your character through packs of enemies. Your Barbarian will automatically attack everything around you.
Repeat: Continue moving and whirling, letting your character's momentum carry you through the dungeon or farming area. Reactivate your Shout periodically to maintain buffs.

There's no complex targeting, no intricate cooldown management beyond your Shout, and no need to cycle through multiple active abilities. This simplicity is the building's greatest strength for farming.
Areas that are ideal for this 2-button build are those with high enemy density and relatively straightforward layouts. Dungeons with linear paths or circular layouts where you can easily pull large groups of enemies together are perfect. Open-world events with large waves of approaching monsters also work well. The build excels where it can constantly be in motion, letting Whirlwind's AoE damage shine against numerous targets.

The efficiency comes from minimizing downtime. Because your interaction is so minimal, you can move almost constantly, engaging new enemies as soon as the previous pack is defeated. This high enemy kill rate per minute directly translates into more loot drops and more Diablo 4 gold picked up. You spend less time standing still casting or managing resources and more time actively killing.
While this build might not be the absolute fastest for killing a single, very high-health boss with complex mechanics (though it can still handle them with sufficient gear), its true power lies in clearing general content and farming for resources.