During Yachatz we break the middle matzah into two slightly uneven parts - one slightly larger than the other. Hashem originally condemned Avraham's descendants into exile in a foreign land for 400 years. But in His great mercy, Hashem cut the years by about half, much as the middle matzah is broken, just about in half. The Jewish nation spent only 210 years in Egypt until the Exodus, instead of the full 400.
(Orchos Chaim)