"It's only after you have to rescue someone you love from a life-or-death situation involving users of this style to know just how much a pain in the ass it really is..."
By incorporating multiple brutal training regimes, the Mandalorian Protectors are able to make their bodies as hard as steel when using the form by locking their muscles and tightening them.
It is debatable whether they are actually taking damage during this period, though from what I can tell from the author and other sources, they physically seem to be impervious to taking damage when using this technique. I have seen images of men with broken wrists and hands after striking a Mandalorian Protector using this style, their bodies simply unbreakable. Whether this extends to even sabres breaking upon contact with their skin, it is unknown; but I am not willing to put it past them.
Now, I reckon certain forms and strengths will be able to breach the defence, that being Teräs Käsi, that of the Knights of Prakith, the Sangheili with their biology, and sufficient Force amplifications. Regardless, the science is clear, with absolute and utter muscle control of the body. Of course, this has the downside of meaning the Protector cannot move, but the author describes that those who take the technique to higher levels are even able to move while using the technique.