The sages instruct us to remove the Sederplate from the table; children will wonder why we are doing this even though we have yet eaten, and they will ask questions. This is not our custom; rather we lift the plate up when we say ha lachma anyaand afterwards we put it back on the table where it originally was. This is an allusion to the waving of the omerup and down in order to avoid the bad dews¹⁴. Why is it necessary to wave it up and down? It should have been enough to simple wave it upward since the dew comes from above. Originally¹⁵ 12b (?) there was a place in the firmament in which the bad as well as the good dew dwelled. David prayed to God that only the good dews should dwell above as we see: “For You are not a God who takes pleasure in wickedness; evil shall not sojourn with You.” (Psalm 5:5) Because of David’s prayers, the wicked dew was stored down below and the dew of blessing above. So when we wave the omerin an upward direction it is to ask that we be blessed with the good dew, and we wave downward it is to ask that God cut off the bad dew. Passover is, of course, the time of year when the world is judged regarding the productivity of the wheat harvest. That is why it was customary to bring an omer of barley at this time of year to the temple. Now that we do not have an altar or a temple, our dinner table is our altar; we reenact this ceremony at the dinner table with the waving of the Sederplate. By lifting up the matzah along with the plate it is meant to mimic the waving of the omeroffering.

footnotes: ¹⁴ There is apparently two types of dew – good dew and bad dew which God has stored away. ¹⁵ Babylonian Talmud Hagigah