I once saw written in a "alon" (given out in shuls before Shabbos) that in regards to eating matzah that it has to be done in the amount of time that is "c'dei achilas pras" -- that this time begins to be counted not from when the food (matzah) enters ones mouth, but rather from the time the person begins to swallow the matzah. The author who wrote this did not mention any source and simply stating that that is the halacha.
What is the source for this? Is it accepted by all poskim or are there those that argue on this? If so, who? Sources?