Before partaking of something you desire, principally food before you eat it and new clothing [before you wear it], you offer it to the Three Jewels. You are slightly going against your desire, because you come second. You don’t get to eat it right away. First you at least think about offering it to the Three Jewels. You still get to eat it, but because you become second, you are confronting desire in a certain way that slightly lessens it. That’s the real reason why we make such offerings.
In addition to that, if you find yourself eating some especially delicious food and you wish to offer the sensation of doing so to the Three Jewels, that’s fine. But you can’t be offering the food because it’s already in your mouth. That would be offering them your leftovers.
[From a teaching on Songs of Barway Dorje, Part 11, by Bardor Tulku Rinpoche. Translated by Lama Yeshe Gyamtso.]