You find that you still have some DETERMINATION within you, more than a normal Monster would but less than a human. Your magic is much more easily summoned, though Alphys touches you on the shoulder and says "There's no need to add your magic to this... this thing's got plenty of electricity and magic in it to do the job."
She places the capsule in, and begins powering it up. Flowey watches as the energy begins to flow from the capsule and into the machine. He gazes warily up at the DT Extractor, giving you the impression he's seen how it's worked previously and isn't so sure about this, but then the thing opens up and fires a beam of arcane energy onto him, bathing him in magic and SOUL energy.
For a moment, nothing seems to happen. Then, slowly, you see Flowey shifting and changing... and can feel it in the air as your own soul and the others come out. You see the outline of an upside down heart from in front of Flowey, and it slowly begins to fill up. With each layer that it is filled, Flowey shifts more and more into his true form as a goat Monster.
Spoiler (click to show/hide):
https://d.facdn.net/art/chelodoy/1572013355/1572013355.chelodoy_asriel_v2.png general look of Asriel, albeit with regular green eyes
Before too long, the transformation has finished, and the DT extractor stops. The light from the magic fades, and you see your big brother Asriel kneeling on the floor, gazing at his hands in near childlike wonder.
You also note, amusingly enough, that he is naked.