While playing of course!
I have been playing with the sao resting against the palm of my hand.
Videos seem to show that the sao is held in between the thumb and pointer finger [ the gap ]
I’ve tried doing this but find it so uncomfortable and then switching back without realization.
Is this absolutely necessary to do while playing, or can I freely hold the sao whatever way is comfortable?
Thanks!
You should definitely maintain proper form. 
Hi ! The sensei there will be able to explain that better than me, but the left hand doesn’t hold the sao. The action and weight of the right arm on the dou (classical aesthetic) must maintain the sao properly without putting weight in the left hand. In tsugaru position, the angle formed by the sao on the right leg must maintain the balance of the shamisen without weight in the left hand either. Let the teachers give us a more reliable explanation !
@Patrick_Younes is correct.
The left hand must be free to move up and down the sao in order to finger the positions. It does not really support the neck.
Here is a video from Fujii Reigen about posture
When I first started playing, I was having a real problem with constantly hitting the wrong string, or sometimes just flat missing the icho-no-ito altogether and only hitting the skin/head. I finally figured out that I was just holding the shamisen wrong, supporting the sao with my hand, rather than balancing the shamisen as others have pointed out. Moving quickly from note to note would cause the whole body of the shamisen to jerk, misdirecting my bachi strike.
When discussing this form problem with my sensei, the inestimable Mike Penny, he told me that when he learned to play in Japan, he wasn’t allowed to touch the shamisen with his left hand for some really long period of time (I can’t remember how long exactly, but it was multiple months). He could only balance the shamisen with his right arm and strike the strings with the bachi. Crazy! But Mike’s form is awesome, so I guess it paid off in the end…
Thank you so much for this valuable information.
I am learning on my own, so things like this are like gold to me, especially since I don’t have someone there to critique me!
I truly appreciate it!
reviving this thread… i have been working on this for weeks with no luck. are you supposed to actually exert some pressure on the dou with your right arm to get it to balance? if i do that, there’s no way i can actually move my arm to play, haha. I’ve tried all angles and combinations I can think of. does anyone have videos of this, with any tips?