Use the straight-arm pulldown to train stubborn lats. For most people, nailing the lats is hard to do. And chin-ups, pull-ups, and pulldowns are more like arm exercises than lat exercises. Likewise, unless you're lat dominant, rowing movements hit mostly the mid-back, not the lats. The straight-arm pulldown solves the problem by removing the biceps and mid-back from involvement and isolates the lats like nothing else.
While the pullover is similar, the cable straight-arm pulldown is superior at targeting the lats because the tension is more constant throughout the range of motion.
The straight-arm pulldown also involves the teres major and posterior deltoids – both good things since they complete the look of the lats and increase the width of your back. These two muscles aren't strong enough to override the lats though, so the lats still take the brunt of the work.
Grip and Torso Angle
Do the straight-arm pulldown with a palms-down grip on the bar. Keep your grip style the same (hands down; pronated), but vary the width–narrower or wider. Change your torso angle for variation. A more upright torso will put more stress on the lower portion of the lats. Lean forward a bit and you'll feel it more in the upper portion and teres major. Try both versions.
Here's the key thing: When doing the straight-arm pulldow, experiment to see which position creates the most tension in the lats. Remember, the goal is not to perform an exercise from point A to point B; it's to use the exercise to load the muscles you want. So the most important thing will always be feeling the lats doing the work.
Once you've mastered the basic Straight-Arm pulldown, you can try the more advanced version which combines both positions. Initiate the pulldown with the torso leaning forward. As you reach the mid-range point, slowly raise your torso to finish upright. If you nail this, you'll feel the strongest possible tension in your lats.
To get this movement down, use a slightly slower speed of movement than usual and focus on keeping the tension on the lats. Try to keep the speed constant during the entire rep. It should take you about two or three seconds to do the concentric (positive) portion of the movement.
The long head of the triceps is also involved and could take away some lat stimulation, so you really have to put your mind in your lats. Only use a weight where you can focus the tension on the lats. Keeping a very slight elbow bend (just enough so that they aren't totally locked) and never changing the elbow angle during the set will also help minimize triceps involvement.