Think it's a rival for a man's affections. If you are married say and another girl comes and tries to take your husband away by competing for his love and affection. The she would be your sautan.Although I think it is normally used when the man is willingly straying. If he feels nothing for her then I don't think it can be used.Not sure if I am 100% right though.
sautan is a women who also married to your husband at the same time