This is an easy one. A playtester asked about essence loss as it applies to magic. He knew the Shadowrun rules, but he didn't see any mention of how it works in Shadowitz, and since the adept rules are different in the two games, he wasn't sure how it worked.
So, here's the explanation, and I'll put it in the book too:
Your magic equals the whole number of your essence. A magically active character with an essence of 6 has a 6 magic. If his essence dips to 5.75, his magic drops to a 5. 3.42 essence = 3 magic.
Now, as to what a dip in magic does to your awakened abilities, that too is a straight equation. Mages need do no further adjustments; sorcery and conjuring mechanics already use a mage's magic score, so the ramifications of the score adjustment are already taken care of. Adepts, on the other hand, must do one more little adjustment. They purchase powers in whole points now, so for each magic point lost, they lose one power, or level of a power. It doesn't have to be the most recently attained one; a player can drop whatever power he likes.
Any awakened character who drops below a 1 essence loses all magical abilities, regardless of how much he's boosted his magic through initiation.