He knew he was sharing more with Jim than he had before, more than was appropriate. The swells in yeswantgoodmore were apparent when they had always been carefully hidden before, and Spock knew this, and tried unsuccessfully to prevent the sharing. Other things took his attention: with each rock in he squirmed again, small movements to try to give himself some mercy to think without distraction...but Jim always seemed to match him move for move.

How will you know I am 'undone'?