Note progression reversed.

[Bax Burgess]

Using hammer-ons, moving fret by fret along an open string, the notes go higher, moving towards the body, and lower towards the headstock. Mute the string over the body with your playing hand, then try the same progression of hammer-ons. What is this?

[bausin]

You're hearing the string vibrating between your finger and the nut. So it becomes lower as you go higher up the neck.

[Bax Burgess]

I just repeated this with Thomastik-Infeld K116 strings and hear two separate notes sounding in unison for each fret when hammered open - without the muting. The thin notes that raise their pitch towards the headstock, heard when muting, are actually present - sounding alongside the bassier open notes, whose pitch is moving lower towards the headstock whilst the thinner notes move higher. I'm unable to hear the paired notes with pb strings.