This is a special way to implemet safety triggers in hypnosis as you know them from other BDSM play: 
The trafic lights red, yellow and green.  
These are the suggestions given in trance.
I want this to be safe and consensual.
Therefore I am giving you your safe words Green, Yellow, and Red.
They work like traffic lights.
You can stop the trance at any time yourself with Red 
when the suggestions leave the ground 
of what you desire deep inside yourself.
Green and all is fine.
Yellow - Your unconscious mind gets alerted to check whether case Red has happened.
Red - That is the safety build in. 
A limit has been crossed and you need to stop that.
Your unconscious mind knows that very well 
because it is protecting you from birth
and also keeps you on track where you should go.
That is a very important task of the unconscious mind 
and we use it here to keep you safe.
Being in trance that is easy. 
You just emerge yourself from trance in case Red.
But to make the emerging a smooth and enjoyable process you emerge with these suggestions
in your mind: 
After I emerge myself from trance I feel fully awake and alert.
After I emerge myself from trance I feel refreshed and comfortable.
After I emerge myself from trance I feel great in all aspects of life.