The story about Mohini Bhasmasura, Bhasmasura was very strong and powerful demon. He was a devotee of the god shiva who performed great penance to obtain a boon from the deity.
Bhasmasura prayed to shiva and pleased with his devotion, shiva decided to grant him a boon. He said, “lord, please grant me the wish that whichever person, place or thing, I place my right hand on, is reduced to ashes.” Lord Shiva said,”Tathastu” and granted the boon. he was very happy.
The wicked Bhasmasura tried the boon on lord shiva himself. Lord shiva started running from him to save himself. To seeing this, lord Vishnu decided to help him. He took the form of mohini and appeared in the front of the demon. Mohini was so beautiful and he was immediately attracted by her.
Bhasmasura requested mohini to marry him. She replied that she would marry him only if he could match her moves in dance. He agreed and started dancing. While dancing, she was placed her hand, especially the forefinger was placed on top of her own head. As Bhasmasura, he touched his own head with his forefinger and was immediately turned into ashes according to the boon which he was gained.
That is how lord Vishnu succeeded in saving shiva from Bhasmasura.