We are HUGE ADVOCATES of the side plank and have highlighted this exercise as well as many variations of it prior. Briefly, the side plank has THE HIGHEST GLUTE MED EMG ACTIVITY OF ANY BODY WEIGHT EXERCISE. The glutes are so important to low back health as well as controlling femoral motion which plays a huge role in [P]Rehab for your knee. Oh yeah, don't forget that this exercise is also a PLANK - meaning we're working on core stabilization as well. Even more benefit for your low back health!
A big problem we see with our athletes is the ability to STABILIZE THEIR CORE WHILE INDEPENDENTLY MOVING THEIR EXTREMITIES. But this dissociation isn't the only problem, there can be some BIG side to side differences in terms of how well an athlete is able to stabilize their core using their left vs right upper or lower extremities (think of your slings).
Do your best to stay as STILL AS POSSIBLE in the side plank position as you move either your arm or leg. Regress by going on a knee. Adding a @theraband (CLX loops) as shown is more facilitory than it is going for any true strength gains.