Noticing change
In order for you to know that you have changed, you have to know you are present.
At every change, your breath changes. You are there to change your partner's breath and your partner is there to change yours. But a prerequisite for you to know that your breath has changed -- that you have changed -- is that you are fully present. You have to be there, before you can know you have been changed.
By the way, in order to be really there on stage, you have to leave everything behind you when you enter it.