The ArHaShulchan 461:7, and the Mishneh Berura 461:13, in discussing the particulars of determining when Matza is baked, make the following comment after mentioning the Shiur of Chutin NimShochim.
Chutin NimShochim means that when the Matza is torn apart, there are no doughy stringy threads stretched between the torn pieces. SEE Following this criterion they say, "and even if when it is moved it crumbles, "Hi NifReChes", one fulfills the Mitzvah of eating Matza since there are no stringy threads."
This appears to make no sense. Clearly the criterion of Matza crumbling is a more comprehensive measure of baking than "no doughy stringy threads" between the torn pieces. If a Matza is baked to the point that it crumbles, it could not possibly have doughy threads. One is tempted to suggest that a slight alteration to the text is in order. Perhaps the words "does not", are missing; i.e. and even though it "does not" crumble when moved, it is nevertheless deemed to be properly baked since there are no stringy threads. The comment might possibly be directed to those who were unfamiliar with soft Matza, reassuring them that it is Kosher.
However, upon closer inspection the meaning is quite simple, we have misunderstood the intent and meaning of "NifReches". The confusion lies in the (mis) translation of the word "NifRaChos". Over time, the meaning has altered; it no longer reflects upon what it used to. Today, we use the word to refer to hard Matza. In modern Hebrew it describes a crunchy cracker. However, the Gemara uses it to refer to Matza that does not retain its form, it easily tears apart.
The Gemara of Menachos 78b, discusses the various meal offerings brought in the Beis HaMikDash, all of which bar two, were Matza, unleavened bread. It quotes a Beraisa that defines when Pesach Matza is adequately baked; when there are no doughy stringy threads stretching between the torn pieces of Matza. Following this definition of baking, Rava declares that the same criterion applies to Matza for Pesach.
The Gemara is troubled by this unnecessary declaration. Rava states the obvious; since both are defined as "Lechem", bread, they are both defined by the same criterion.
However, the Gemara explains: since the sacrificial breads must be whole and complete, we may have followed a Talmudic principle of, "Kol HaOmed". According to this principle, if we for example, are instructed to combine certain ingredients, we need not actually combine them provided we have them in a container which is large enough to allow us to mix them without spilling. Similarly here, although the finished product is whole and complete, it will not maintain that completeness if we were to pick it up by one edge and move it. It is not baked so thoroughly to be firm and remain complete if moved by an edge. Rava declares that this is not a problem, it is still deemed to be complete and whole until it actually is broken.
BTW, this is the determinant of when a loaf is complete for being used as Lechem Mishna. If a loaf has been partially cut into it may still be deemed complete and used as Lechem Mishna. Provided the loaf can be handled by its smaller part and not break apart, it is deemed to be complete.
We have therefore various levels of baking. Chutin NimShochim, when there are no doughy stringy threads, is the first level of baking. A further step is attained when the baked goods can be handled by their edge and moved around without breaking apart.
The Mishneh Berura and other Poskei Acharonim are simply saying that the Matza is satisfactorily baked even though it will break apart when handled by their edge and moved around.