Mikvaos, Chapter 10, Mishnah 4. Chatzitzah is the theme of this chapter: whatever interposes itself between a mikveh's waters and the item being toiveled. Our Mishnah develops that theme along two lines. It first lists the knots on clothing that have to be opened before the tevilah, and it then teaches the correct way to toivel a garment itself.
Knots that water must reach
The Mishnah heads its list with the words "tzerichim sheyavo vahem mayim", meaning that these are the places where the water is required to arrive. Practically speaking, such knots must be undone so nothing prevents the water from getting in, and the reason is that they belong to the category of knots a person does not leave tied forever. Where a knot is routinely fastened and unfastened, it never acquires the status of part of the garment itself, and for as long as it stays tied it obstructs the water and constitutes a chatzitzah.
The first case: "hakesher shebeparkason", the knot on an undergarment. Since a person unties it whenever he removes the garment, the straps are not left tied as a permanent arrangement. Therefore, as long as the knot remains tied it is a chatzitzah, and it must be opened before the tevilah.
Next, "shebekasaf", the straps that pass over the shoulder. The same logic applies to them.
The Mishnah adds a case that is not a knot at all but a fold: "vesofo shel sadin", the edge of a sheet, which naturally curls over on itself. "Tzarich limatoch", one must draw it out and straighten it, because those folds and bends are places into which the water is required to reach.
Then the Mishnah turns to tefillin. "Utefillah shel rosh bizman she'einah chogeres", the straps of the tefillin shel rosh, when they are not drawn tight, have to be loosened before tevilah. "Veshel zero'a bizman shehi olah veyoredes", and likewise the knot of the tefillin shel yad, when the strap is not tied firmly but slides up and down, must be loosened.
Finally, "ushnatzos shel sandal", the laces and straps of a shoe. These are tied and untied every time the shoe is put on and taken off, so they are certainly not left in a fixed tied state. They too must be opened at the time of tevilah so that the water can reach into those places.
Toiveling a garment: the air inside the fabric
The Mishnah now moves to the tevilah of a beged. Just as a keli has air inside it that must be displaced by the water of the mikveh, a garment also holds air, in the spaces between the threads. That air has to leave the fabric entirely so that the water can penetrate everywhere.
The Mishnah now takes up the tevilah of a beged. A keli contains air within it, and that air has to be pushed out and replaced by the mikveh's water; the same is true of a garment, whose weave holds pockets of air among the threads. All of that trapped air must be driven out of the cloth so that nothing remains dry and the water reaches into every last spot.
But "hitbilan neguvim", if he toivels them dry: there the standard is "ad sheyivbe'u veyanuchu mibibuyan", one must wait until the bubbling stops. The bubbles are the air escaping from the fabric; when they cease, that shows that the air has all left and the garment is fully saturated with the water of the mikveh. Only then is it a proper ma'aseh tevilah.
The thread running through the whole Mishnah is a single one: the water of the mikveh must have unobstructed access to every part of the object. A knot that a person routinely unties, a sheet's curled hem, or a pocket of trapped air in the weave of a garment all stand in the water's way, and each is dealt with in its own manner.