As I prepare for my Intro 2 assessment, one thing I noticed was that my arms, as well as my student's arms, move sideways while doing Parsva Halasana. Of course, when you do Parsva Halasana, you can't see your arms - even if you are doing yoga in a place with mirrors. Unless the mirrors are on the ceiling... which, um, I would question the wisdom of doing yoga in such a place.
Anyway, how do you know where your arms are drifting? One way is to develop such depth and awareness in your practice so you can feel slight movements in your arms, while upside down, turning your spine, pressing your upper arms into the floor, softening your neck, leveling your hips, straightening your legs...
I don't doubt there are people out there with such awareness. I'm not one of them. I use a strap and a brick instead.
Here's how I set up: a brick tall enough to feel between the hands and a strap around the wrists. This picture tells me my left arm (the one with the watch) is further away from the midline than my right. (Not something I could tell from this setup. I would have to use a bigger brick.)
This arm position is in Halasana:
Now I move to Parsva Halasana to my right:
And my arms move to my left. I touch the brick with my right hand:
Now I know I need to move my arms:
Even with this knowledge, when I take my legs to the left, the same thing happens, even with Iggy's help:
For more information on what it means to be a certified Iyengar teacher, check out the following links:
BKS Iyengar official site