There is so little that can be determined by playing with hair, mostly beacuse the attack angle determines how well it will cut. Essentially, there is technique to this as well as the knife being sharp, so it's really far from an objective test in the least. You can spend all day doing HHTs, to whittling hairs, to trying to whittle strands off the strands you just whittled, it doesn't mean anything about your edge except that it's sharp enough to handle ANYTHING else you throw at it. In the meantime, it can be so tricky to actually get the hair in the right position to cut that it can often times be more a matter of technique than anything else. It's a parlor trick. And I'm not even mentioning the differences in hardness, thickness, moisture content, curly or straight, so on and so forth.
Personally I find buckman110's test more useful. If I pick up a knife and it can't whtitle hair... Is it beacuse my hair is too dry, too curly, not straight enough, so on and so forth, or is it because the knife is dull? If I pick up a knife that can't slice into my fingerprint like that, the edge is dull. It's just that simple. Beyond that I can usually gauge how quickly my edges dull by how easily they slice into my thumbpad compared to when I first started using the edge. It's much more useful since it's way more consistent. I think it's pretty similar to Murray Carter's three-finger test.
Honestly I've not been sharpening knives "seriously" for some ten or twenty years to be able to espouse my expert knowledge. But honestly, I can't see it taking twenty years to realize how much usefulness this test really lacks. It might be nice to see if you've reached some higher level of sharpness in an acamedic setting--you know just to see if you can. Otherwise there's no practical benefit because there's no way to percieve these different levesl of sharpness without relying on completely abstract testing methods like this. If it whittles hair, or if it shaves hair, it's going to cut a box just as well.
Do you know why straight-razor enthusiasts spend soo much time worrying about the hair-hanging test? Because they're using it to shave hair.
Oh, and uhh, the obligatory proof
You can even see it hanging onto the edge by one of the strands whittled...
But even that picture is meaningless to me. You know why? Because when I first heard of hair whittling, I use to stretch a hair out between two fingers so it was taught and do it. I thought "Hey that's hair whittling." It takes a lot more refining to hold a hair out unsupported to do it. But you know how much discernable difference in sharpness there is between those two levels in actual use? None, none at all.
Determining an edge sharpness based on whether it bisects the hair or simply carves strands off it is just splitting hairs. Pun intended.
