Commentaire sur Mikvaot 10:4

פרקסין (underwear/shirt) – the underclothes that are upon the skin, and this is open at the shoulders, and when he wears it, he ties it on the shoulder, and at the time of removal/stripping, he loosens it.

The following require that water shall enter inside them:
The knot in an undergarment which is tied to the shoulder.
The hem of a sheet must be stretched out.
And the knot of head tefillin if it is not fastened tightly,
Or of the arm-tefillin if it moves up and down.
And the laces of a sandal.
Clothes which are immersed when they have just been washed must be kept immersed until they bubble up; But if they are immersed when already dry, they must be kept immersed until they throw up bubbles and then cease to bubble up.
Today's mishnah is the opposite of yesterday's. It deals with situations in which water does need to come into contact with the insides of knots or other such cases.
Section one: The undergarment was attached to the shoulders by a knot. Since this knot would regularly be undone, it needs to allow water inside when immersed.
Section two: Before one immerses the hem of a sheet which has wrinkles and folds in it, one must stretch it out so that the water comes into contact with every part.
Sections three and four: These were explained in yesterday's mishnah.
Section five: While the thong of a sandal did not need to let the water inside it, the laces are regularly undone. Therefore, they need to be undone to allow water in and out.
Section six: The mishnah requires a sign that the water has come into contact with the clothes for the immersion to be efective. If the clothes are already wet, then all that needs to happen is for the clothes to sort of bubble up, meaning they pop up to the surface.
If they are already dry, then the clothes must first bubble up and then stop bubbling up to the surface. Only this is a sign that they have been successfully immersed.

ושפה של סדין צריך למתה – the folds/creases that are in the upper rim/hem in order that the water can come between those folds/creases.

למתח (to stretch) – to stretch them and to draw them out/extend them, like (Isaiah 40:22): “Stretched them out like a tent to dwell in.”

ושנץ של סנדל (laces of a sandal) – straps that one ties/knots them.

עד שיבעבעו (of garments dipped in water until they are soaked thru and cease from bulging) – when a person immerses clothing in the water, they emerge like kinds of bubbles/bulges and when they are launder and liquid drips from them, when the water begins to bulge, they have been purified, because they have been in contact with the water that has been absorbed by them to the waters of the Mikveh, but when they are dried, the water does not come into all of it until they have ceased from bulging. That when a person immerses a sheet and is not able to stretch it out, for the Mikveh is not that wide, he inserts it in the water when it is folded up, and through it the water bulges/bubbles, but when they rest from bulging it is known that the waters came through all of it.