To say it simply, the Supreme throttles Himself down to become us, become human. Just as a big, huge nuclear generator throttles itself down to become electricity that is found in a light bulb , giving off light. It is the same śakti (energy) just stepped down into the form that can be used by the light bulb to give off its brilliance.
In the pratyavbhijñāhṛdayaṁ¹ āgama there is a wonderful insight that is offered in sūtra 7 , it says: and (though) He (śiva-bhaṭṭārka) is one He becomes of two-fold form, threefold, fourfold and of the nature of sapta and pañca ( or 5 x7 =35).
This is the notion that the Supreme throttles down into creation . The following post reviews this knowledge http://www.hindudharmaforums.com/showthread.php?t=6649
Pratyabhijñāhṛdayaṁ is authored by kṣemarāja-ji śiṣya of abhinavagupta. This word Pratyabhijñāhṛdayaṁ means the re-recognition of ones Self - in this word hṛdya is used as ~heart~ or the inner most , most dear, and hence the Self.