There are many variations and modifications for this pose, but I choose to teach it in the classic Ashtanga sense. This style makes the pose reminiscent of Triangle (Trikonasana), and a few other preparatory postures.
Here's what typically goes through my mind when I'm practicing Side Plank B:
1. Is my bottom hand anchored to the ground?
The hand should be well-positioned so that the wrist fold is parallel to the front of the mat. You can also try turning the hand outward a few degrees if that helps take pressure off your wrist. Once stabilized, think about pressing the circumference of the palm down.
2. Am I lengthening through my spine?
There really isn't much difference in the spine between Mountain Pose (Tadasana) and this pose. I create the same effort in the upper back to prevent any over-rounding in the spine. Aiming the tailbone toward the heels will keep your lower back from flattening or overarching.
It's easy to crunch the lower side body and laterally tilt the spine, but that prevents the stability that comes from using the side body just enough to keep the waist even from left to right.
3. Are my hips square?
It's super easy to accidentally torque from the wrong place, so I'm always looking down to make sure my hips are stacked and my pelvis is square before I start to address lifting the top leg.
4. Is my lifted leg externally rotated?
When you start to lift the top leg off the bottom, externally rotate it so that the knee and toes start to turn up toward the ceiling. I also teach that the foot will not reach straight up to the sky over the bottom leg, but rather, that it will angle toward the side of the room you're facing.
5. Is my bottom shoulder well-positioned and stabilized?
Keeping my palm anchored, the aim is to turn the bicep to face the front of the mat so the shoulder is moving in slight external rotation. This will help combat its usual desire to internally rotate.
6. Are my muscles activated to protect my elbow?
For me, this tends to feel like my elbow having a fight with itself. I think about firming the bicep and tricep simultaneously, as though my elbow was fighting to bend and straighten at the same time.
And most importantly, I always remind myself and my students that grabbing the top toe doesn't happen until all of the other pieces of the pose have come into place!
Photo courtesy of Oliver Endahl, Graphic design by mbg creative