The side plank with dumbbell external rotation is a movement that combines an isometric side plank hold with a dumbbell rotation to target both the core and the rotator cuff muscles. You probably won't be able to use much weight without disrupting your balance, but even at light weight, this is a movement that can help address multiple muscle groups in a dynamic warm-up or as accessory work at the end of a workout.
Can improve rotator cuff strength as prehab or as rehab after injury
Improved shoulder external rotation carries over to better performance and lifting technique
Side plank position strengthens the core, arms, shoulders, and legs