One arm up, core fighting the tilt.
Resist leaning to the side. Keep your torso stacked so your obliques work to stay tall.
Pressing one dumbbell at a time makes the load lopsided on purpose. The working delt drives the weight overhead with the triceps finishing the lockout, while the whole trunk fights the sideways pull that one-sided loading creates. Each arm takes its turn with no help from the other, which makes it both a shoulder builder and a stability drill in one movement.
The uneven load forces your trunk to resist tipping toward the loaded side, and each shoulder works with zero help or rhythm from the other. You build pressing strength and the stability to use it at the same time.
Start slightly lighter. The anti-lean demand eats into your pressing strength at first, and a clean vertical press matters more than matching your usual number. The load catches up as your trunk gets used to the job.
Finish all reps on one side, then switch, as the movement is designed. Starting with the weaker arm while you are fresh, then matching those reps on the stronger side, is the standard way to keep the sides even.
