On the seder night we do yahcatz before we begin magid, i.e. we break the middle matza in half, and put away the larger half for the afikoman, before we tell the story of yetsias Mitzrayim. The Talmud understood that the matza on Pesach night is called "lechem oni" for two reasons: 1) the matzas should be on the table while we tell the story of yetsias Mitzrayim ("lechem sheonim alav devarim harbey") 2) that the matza should be a prusa (broken and not whole). These two reasons combined cause us to have the broken matza (prusa) on the table ("lechem sheonim…") before magid.
We find that the same matza is symbolic of both the slavery of our ancestors as well as their being freed (the geula). Immediately following yachatz we declare "ho lachma anya", that this type of matza was eaten by our forefathers in Egypt, and is therefore referred to by the Torah as "lechem oni". Later on in the hagada we quote the statement of Rabban Gamliel that the matza we eat is reminiscent of the fact that at the time of the geula the Jews left Egypt in such haste that there wasn't enough time for their dough to rise. Why have the matza symbolize two opposite concepts?
The mishna (Berachos 54a) tells us that just as one recites a beracha to praise Hashem when something wonderful occurs, so too we ought to recite a beracha praising Hashem when a tragedy occurs. The wording of the mishna ("just like…so too") seems to equate the two berachos. The Talmud (60b) finds this equation difficult, since in fact the beracha we recite on good tidings (hatov vehamaitiv) is different than the beracha we recite when a tragedy occurs (dayan hamemes). This equation seems no more valid than saying that just as one recites a beracha upon eating potato chips (hoadama), so too one should recite a beracha upon putting on tefillin (l'haniach Tefillin) - the two berachos recited on the two occasions have nothing to with each other!
The gemara answers the question by explaining that while the wording of the two berachos is different, they do in fact share something common: both should be recited in a state of joy (simcha). Why should both be recited b'simcha? The Shulchan Aruch quotes from Rabbeinu Yona that we ought to have emunah (faith) that everything that G-d does is really for the good. While the wording (nusach) of the beracha cannot be the same, since according to our perception we experience a tragedy, at the same time we are expected to believe that an awful tragedy is really letova and that G-d would never allow anything that is objectively bad to occur. Therefore we recite the bearcha of dayan haemes in a state of simcha.
At yachatz one matza is broken into two parts. The appearance at first is that the matza represents the pain and suffering that our forefathers experienced while in slavery. But we refer to the afikoman as tzafun (the hidden piece). What was concealed from us during all the years of slavery was that all of that suffering was really letova. The two pieces are from the same matza! Just as the second half-matza representing geula was clearly letova, so too the first half-matza representing the years of pain and suffering was also letova.