(I posted an abbreviated version of this on another forum - B&B (gasp!) some time ago....)
I agree that the HHT is not an infallible test - I have had good shaves from razors that wouldn't pass the HHT, but - in common with a lot of people - I have never had a bad shave from one that did pass the HHT. I think the ability to pass HHT is due to both the blade and the hair being used.
If hone a razor on a coticule and try HHT - probably it will not pass (I'm talking about straightforward honing, no drawn-out dilution methods and not using a tape for the final bevel). I can strop the blade and it may or may not pass. If I use CrOx before stropping, it will pass. It is probably well-known that the much-vaunted smooth shave given by a coticule is a function of how it rounds the edge of the blade slightly (putting it simply), so this smoothing of the cutting edge has a direct influence on the HHT (once again, I'm talking about straightforward honing on a coticule - as far as I am concerned, the last thing that refines the blade is the edge you are feeling - if you use CrOx after a coticule then you are experiencing the CrOx edge, not the edge left by the coticule).
Perhaps the main contributing factor as to whether the HHT is passed or not is the hair itself- hair may look like very even and glassy-smooth to the eye, but the microscope shows it to be covered in scales, much like the bark of a monkey-puzzle tree. The scales lay in one direction - if an edge is moved along the direction of growth (ie from scalp to end of hair) it would meet no resistance, if it moved the other way it would snag on each scale. If the edge in question is the edge of a razor, then to be able to snag it must be fine enough to push against the raised edge of the scale, so a blade honed purely on a coticule for instance is usually too rounded (without further refinement) to feel the resistance and does not snag. It doesn't mean that it will not give a good, satisfying shave though in much the same way as an axe laid gently on your neck won't cut it - but it's still capable of chopping your head off!
To increase the chances of passing the HHT, the 'bulb' or root of the hair should be facing away from you so the downward action of the hand holding the hair brings the upraised edges of the scales into contact with the blade. The test is performed by slowly bringing the hair down onto the blade and following through - not by forcing the hair, moving it quickly, slicing with it, etc.
Everyones hair is different - the scales ("cuticle" technically) on one persons hair may be very fine and even and not stand out much:
while another persons may be in less good condition:
You can see that a razor's edge would easily snag against the upraised scales in the second photo. IMO the serrations on the razors edge have nothing to do with it - it's the scales on the hair that matter and whether the bevel is slightly rounded or very acute. Lots of things affect the success of passing the HHT: the type of hone, how much slack given when stropping (ie stropping rounds the edge), the type of strop you use (paddles do not round to the same degree as hanging strops), your hair type, the condition your hair is in, the humidity (the scales become less prominent as humidity rises), etc, etc. Some people say that the hair should be tested before stropping, as stropping increases the chances of passing - so what? We are only after a decent shave after all, not a set of self-imposed rituals.
To some up with, the HHT is not fool-proof and can even be inconsistent with the same persons hairs on different days, but coupled with a shave test it can soon be a good indication of how the razor will shave and be a ggod benchmark for a honer. Th ultimate test is the shave test though - actually shaving with the razor. As stated before, some razors that do not pass HHT give outstanding shaves.
All this is just my personal opinion though - YMMV.
Regards,
Neil