It is very easy and even tempting to use these buttons destructively. To imply a sub is selfish for considering her needs is an attempt to imply guilt to avoid dealing with real issues. A lot is said within those 3 words. Safe is fairly clear. Sane means we don't do things to screw up with each others heads and we wont do things to each other that will land one of us in a psychiatric ward, ICU or the grave.
Consent however needs some discussion.
Subs must relate the existence of phobias to their Dominants. During the opening up process of negotiation, minor fears and uncertainties must be revealed for the safety, sanity and happiness of both partners. There are physical issues to discuss too , the Dominant would rather know about your weak back. Establish safe words and signals WELL before the scene begins. A word to slow things down are common.