It's Not a Lawnmower!
Dumbbell rows are an excellent exercise, but most lifters look like they're trying to pull-start a lawn mower when they do them. This severely limits the bang-for-your buck benefits of single-arm rows because torso rotation replaces scapular retraction as the primary mover of the weight.
To maximize reward and minimize risk, adopt a split stance and stand perpendicular to the bench.
Dumbbell Split Row
This slight change allows you to row huge weight for strength and mass while significantly increasing the anti-rotation and anti-flexion demands of the typical row. This builds an injury-resistant trunk while allowing you to hoist heavy weights for better pulling strength.
How to Do It
With a heavy dumbbell in one hand, lock the opposite arm out on a bench and hold a flat back position. Keep the feet slightly staggered and row the dumbbell up towards your ribcage while preventing your torso from twisting and rotating.