So, I recently finished reading a book by Kenneth Grant that I got from my mother-in-law last Christmas. I generally don't like Grant's writing style but I have to admit he certainly knew both Crowley and Spare. In the chapter about Spare I have actually found a description of one death posture that was used by Spare.
It was said that Spare used one hand to cover his mouth and nose (pinching the nose with the thumb) thus preventing the oxygen to come in or out while he was drawing the sigil with the other hand, focusing fully on each line of the sigil.
This find is very interesting, not only because usually during a posture you focus on a finished product, but also because my wife uses a similar method, she usually keeps redrawing the sigil over and over again (deepening those lines with each repetition) while chanting the sigil mantra prepared for that very sigil.