Fascinating...Parivrtta Trikonsana (Revolved Triangle Pose) is considered the counterpose to Trikonasana in the book. Frankly, I am not sure why it is. Something Judith Lasater once said about counter poses has always stuck with me. You should always feel complete after a pose. Each pose should leave your body feeling great and done. If you feel your body really needs to move into another pose, then something was wrong with the original pose.
As I have considered her idea over the years, I want to add that I feel she must have meant physically complete because energetically we do need to do counters to balance the energy. When we do something big, we want to counter with something smaller. When you go back, we want to go forward, etc. So, not sure why Par Trik is the counter to Trik here...looking forward to playing with that one more!
I also practiced the arm balance Dwi Hasta Bhujasana (Two Arms Pinched Pose #125). I discovered I have never straightened my arms fully. I also discovered why -- it is much harder with the arms straight! My legs worked much more, pinching the arms, as did my core. Very enlightening! I think I just sat down on my arms (and was quite comfortable!)