Sit on a bench with your legs apart, holding a dumbbell in one hand. Brace the back of your upper arm against the inside of your same-side thigh — that contact point is what isolates the movement, since your arm can't swing or borrow momentum from your shoulder. Curl the weight up toward your shoulder, squeeze, then lower it fully until your arm is straight. Lean your torso forward slightly from your hips rather than hunching your back to make room for the curl. Because your upper arm is locked against your leg, this version leaves nowhere to cheat with body swing — a slow, deliberate curl is the only way to move the weight.