The analogy I use a lot of times is the passing of the Olympic Flame from runner to runner.
Perhaps I'm wrong, but my opinion is that the "nature" of transmission is selflessness. The emptying of one receptacle into the openness of another, both the pouring out and the making room being acts of selflessness (the emptiness nature of things).
Dzongsar Khyentse Rinpoche said in a question and answer session in Bhutan (from an online video of the event) that empowerment is like introducing a person with amnesia to their actual identity. Your actual name is Joe (or Vajrasattva) and these are your things, etc. It can take a person a while to get used to that. However this is the case you really are Vajrasattva or Avalokiteshvara or Tara, etc. ultimately because you have Buddha Nature. However, your Buddha Nature has to be reawakened and developed.
The person bestowing the empowerment has to also meet specific requirements. For one thing they have to have practiced the actual practice and they are supposed to have received signs as well (actual results that manifest in specific ways). Additionally they are transmitting the blessings of the lineage. Vajrayana and Dzogchen operate on the basis of blessing and this is a real factor.
_________________Kirt's Tibetan Translation Notes
"Only you can make your mind beautiful."
HH Chetsang Rinpoche