A rotating press that hits every head of the shoulder.
Make the rotation smooth and continuous, don't rush the turn at the bottom.
The Arnold press starts palms in at your chest and rotates to palms forward at lockout, sweeping the dumbbells through a bigger arc than a standard press. That rotation stretches the front delts hard at the bottom and drags the side delts into the work through the middle, with the triceps closing out the lockout. More range, more time under tension, more shoulder per rep.
The rotation. A standard dumbbell press starts and finishes palms forward, while the Arnold begins palms facing you and turns through the rep. That extra arc works the delts through more range but costs you some load.
Totally normal. The rotating path is longer and less stable, so the same dumbbells feel heavier. Judge it on the stimulus, not the number, and keep a weight you can rotate smoothly.
Seated is the standard, it locks your torso in so the shoulders do all the work and the rotation stays controlled. Standing brings your trunk into it but usually costs strictness. Start seated.
