The mishnah says: "ein maftirin achar hapesach afikomen" you shouldn't eat anything after the afikomen. That goes on till the time of eating the afikomen has passed. The time of eating the afikomen is till chatzos/midnight (which should be observed at least for the first night) but according to some shitos, including the ramba"n, its till the alos hashachar/sunrise. The last piece of matzah you eat is considered as afikomen. If, as it usually happens, you finished the hagadah, and its a few min to chatzos, and youre not ready to eat the afikomen cuz you still wanna eat the seudah, (which you can't do after afikomen) there's a grand idea in shu"t avnai nezer: take a piece of matzah and say: "if the halacha is in accordance with the shitos that the time is till chatzos, than this piece is the afikomen and my last piece of matzah before chatzos" so then, he can't eat anything till chatzos, but after chatzos he can keep on with the seudah since its no longer the time of eating the afikomen!, "and if the halacha is in accordance with the shita that the time is till the morning, than this is just plain piece of matzah" and he can keep up hith the seudah right away, since he still have plenty of time till the morning to eat the afikomen! And according to Briska Ruv, there's no need to "say" the t'nai, because the last piece of matzah is automatically considered as afikomen, no matter what he says. So if the hlacha is with shita #1 than THIS is his afikomen, since its the last piece of matzah before chatzos, and if the halacha is with shita #2, than this is NOT his afikomen, because he's still going to eat matzah before the alos hashachar!