(I intend to write an official introduction soon, but figured I'd offer my two cents anyway...)
I wouldn't go so far as to say that Phowa is the quintessence of Chod. Phowa is the catalyst, absolutely, because it is the method by which we actually abandon our body and sever our clinging to it. However, in most Phowa practices found in Chod, our consciousness is transferred into Troma Nagmo or Vajravarahi by means of "Sambhogakaya Phowa," which is still utilizing a dualistic perspective, albeit pure. The essence of Chod is found in the union with emptiness, however. The jewel of Machig's lineage was in her teachings on Mahamudra, and on Dzogchen (found through Terma revelation), specifically through the Ultimate Phowa of uniting with the Great Mother, basic space.
In Dzogchen Chod, it is often remarked that "tar chod ja dang chod je chod yul kun, she dzogpa chenpor macho AH." "In the end, the offerer, the act of offering, and the offering field, all of these, dissolve into the unfabricated Great Perfection (Dzogpachenpo) AH." This is the true essence of Chod. Machig herself developed a supreme Phowa through the Opening the Gate to Space practice (from Karma Chagme), which is the practice through which the Great Chod Empowerment is transmitted.
The supreme Phowa of Machig does go into some depth speaking about the separation of the mind from the body, and through the severance of attachment to the body, merging both completely with the enlightened body of the Great Mother. Dharmakaya.
More logistically, however, Phowa techniques become increasingly important with such Chod cycles as the Kagyu LuJin, where a full 7-step Phowa is utilized. Otherwise, it is generally a short Phowa, often signaled by a PHAT. But no matter how brief the moment is during the ritual, it is of vital importance for the entire practice. The actual severance of ego-clinging occurs when we leave our body, along with our identity, friends, emotions, memories, possessions, etc. behind and feed it to the various hosts of beings. But the most profound Phowa in the Chod tradition is the unification with the Great Mother.