The regular one-arm row is fine, but this decline variation will better target the lats, especially the lower portion. The decline angle will favor a stronger peak contraction by keeping the tension on the lats even at the end of the concentric (lifting) portion of the movement.
How to Do It
With your knee on a decline bench, start like you would for a one-arm dumbbell row, only now your hips will be higher than your head.
Begin with the dumbbell in front of you, arm stretched out, rather than straight below your torso.
Instead of pulling straight up, bring the dumbbell toward your hip in an arc motion. The elbow should be slightly bent to avoid the overuse of the biceps.