At the time of slaying Hiranyakasipu it was estimated that power of Vishnu alone was insufficient to counter the Rakshas due many clever boons he received from Brahma to ensure impossibility of his own death. So the amsa of Shiva had to be added to Vishnu's active Avatar for the attack.
How is the power invoked for transference ? By prayer? manthras of yagnya or thanthra? by voluntary intention from a higher power source? Or is it by request for action through the power of Bhakthi like Prahlad's towards Vishnu? Or Gajendra's to Vishnu?
The power of Bhakthi is a strong belief even in modern times, so this question.