You can think of surrender as reducing a certain kind of tension: the tension over which you have deliberate voluntary control. Anything that happens automatically and involuntarily in the mind and body is just fine -- only that which feels like YOUR burden, YOUR decision -- in the body, in the attention, and in the mental decisions of the mind -- should be relaxed. The deliberate, voluntary grasping should be reduced. While grasping has an object, relaxation has none. It should also be clear that relaxation is not about preventing any particular thought or action from occurring -- any kind of rigid prevention of anything would simply be a tension in the other direction.
In this relaxation of voluntary tension... a relative quiet is created. If that quiet is vigilantly maintained without the mind going to sleep, and if one asks intensely and continuously how the surrender may be completed, the door will open and the Truth will become apparent: that there was never anyone to deliberately, voluntarily control anything.