During many back exercises, your forearms give out before your back muscles, especially when using a slower tempo for hypertrophy. With the "sidewinder" method, you'll squeeze every bit of tension out of the lats before your forearms crap out.
The sidewinder row is also great for mitigating overextension of the upper arm. This decreases or eliminates anterior shearing force on the shoulder and gets maximum engagement of the lats.
Here's how it looks with various implements:
How It Hits The Lats
The latissimus dorsi has a few key actions: adduction and extension of the arm, hyperextension of the arm, and internal rotation of the arm. And if the arm bone is fixed against the scapula, it pulls the pectoral girdle back as a whole. This row taps into all of these actions.
What To Do
Set up as you would for any row variation, but start with your thumb turned in, internally rotating the arm. This will create tension on the lats right away.
Lead with your elbow and shoulder blade. Pull the elbow to your side while maintaining tension and externally rotating the arm.
By externally rotating the arm and bringing the thumb to point laterally, your elbow will automatically stop at the point of maximum engagement.
If you're freestanding, rotate your ribcage by about 30 degrees to finish the contraction.
As you lower the weight back down, spiral the arm back into internal rotation while maintaining constant tension.
Sets, Reps, and Tempo
Do 3-5 sets of 8-15 reps and experiment with tempo, making sure to contract the muscle and control the movement. Maintain constant time under tension and never let up until the set is complete.