I think it has something to do with her body, it seems her drool is like a drug, they actually do seem to mention it, perhaps that is the mystery behind her, she was so excited that the drool she gave him had all that excitement in it and transferred it to him. Her ability to read his dreams is a new thing but it may be related to her mystery, she is definitely weird but on another look she is quite attractive.
We need to hope that it gets explained in more detail, but for now this is my best shot, she was naked and extremely excited about what would have happened if he broke that promise and looked. She said it herself she was actually more scared he would break the promise and that was thrilling for her and made her heart beat faster, excitement is not always an erotic feeling, sometimes adrenaline makes you excited for different reasons like fight or fear.
We just need to hope they explain this in more detail.