Most lifters have heard of 21s. This intensity technique is most often seen with barbell curls. You do 7 partial reps in only the bottom range of motion, 7 partials in the upper range, and then 7 full-range reps.
But that's not the only way to use 21s. Instead of training different parts of the range of motion, you can also change grips every 7 reps. Here's how it looks. Note the grip changes:
Perform on a low-cable machine with a 30-degree incline bench. Do 7 reps with three different grips: pronated, neutral, and supinated.
Keep your chin tucked and make sure you set up your bench far enough away to maintain tension on the cable at all times. Your weight stack shouldn't "clank" on each rep. You want full tension throughout the set.
This exercise works the lattismus, rhomboids, and posterior delts, as well as the traps to varying degrees due to the variation of grip. Additionally, the chest-supported variation aids in effectively targeting the musculature of the upper back, allowing your midline to stay static throughout range of motion (less chance of cheating.)
This can be done for 3-5 sets, performing 7 reps of each grip variation with 60-90 seconds of rest between sets. Loading should be light.