One would do well to remember that the story described here is a popular folk tale and not described or hinted in any of the puranas. The puranic interpretation is as follows:
Once, Parvati asks Shiva to chose the one form among her 10 forms which he likes most. To her surprise, Shiva reveals that he is most comfortable with her Kali form, in which she is bereft of her jewellery, her human-form, her clothes, her emotions and where she is only raw, chaotic energy, where she is as terrible as time itself and even greater than time.As Parvati takes the form of Kali, Shiva lies at her feet and requests her to place her foot on his chest, upon his heart.Once in this form, Shiva requests her to have this place, below her feet in her iconic image which would be worshiped throughout.