Actually, I DO know about this one…and from personal experience! 
I do not advise using a fist that strikes with the last two knuckles to strike hard parts of the body or hard objects. I have twice struck hard objects (the first time an automobile and the second a refrigerator) in a fit of rage, and both times broken parts of my hand. Striking the van, I used the punch from the end of the thirds of the Taiji form, only driving instead of snapping, and broke several of the bones in the little finger side of my hand. It swelled up and although my jow took that and the horrible bruising away, the intense pain and grinding bones took months to heal. The second time, I struck an old metal refrigerator with that same hand, but with a roundhouse punch. I left my fist’s imprint in the side of the thing, but broke my pinkie knuckle.
I learned several things from these two encounters with inanimate objects. First, I’m never going to do it again. Second, I am quite capable of moving 1000 lbs (literally) with a single punch. (The van actually rocked forward, and if it were in Neutral, may have rolled.) Third, the only thing I accomplished by expressing anger in such a way was breaking my hand, nothing at all positive about that, unless you count the realization that I’m not going through that again.
I have hit objects before with fa-jing, with my hand rebounding off the stone or whatever with no damage or scrapes. So it is possible to strike hard objects with fa-jing without sustaining damage. However, these times I meant to drive my hand right through, as if I were trying to drive my hand right through a person.
I have also chipped bones in my hand doing Bagua post striking exercises. And I wasn’t even trying to hit hard! I just hit at a wrong angle.
In any case, I learned the hard way Yang Lu-chan’s lesson about how the only men Taiji cannot defeat are made of iron, stone and wood.
On the upside, my right hand has fused back together fairly well, and I am of the opinion that, after testing the structure through striking, my right hand is actually now more stable. It actually feels more solid now.
But still, don’t use those knuckles for striking hard parts of the bod. You can get injured.