Rotate as you press and hit every head of the delt.
Keep the rotation smooth and don't let your lower back arch at lockout.
Starting with palms facing you and rotating them forward as you press takes the delts through a longer arc than a standard overhead press, sweeping the front and side of the shoulder through the movement instead of just pushing straight up. The triceps finish the lockout, and doing it standing means your whole body has to hold position while the arc unfolds. It is the pick when you want maximum shoulder range from a pair of dumbbells rather than the shortest path overhead.
The Arnold adds a rotation from palms-in to palms-forward, lengthening the arc and sweeping more of the delt through the rep. The standard press is a shorter, stronger path, so use it for heavy work and the Arnold for range and time under tension.
Standing removes the back support, so you have to control the rotation without leaning into a bench, which keeps the movement honest. The trade-off is slightly less weight, which is fine, this is not the lift to ego-load.
Squeeze your glutes and brace before each rep, and lower the weight if the arch persists. An arching back means the load is beyond what you can press strictly while standing.
