The classic reduction of harmine to THH uses metallic sodium dissolving in amyl alcohol (1-pentanol). This is a dissolving metal just like the zinc or magnesium reduction of DHH; sodium is a more powerful reductant - it's reactive enough to bring about this "hydrogenation".
The idea of "nascent hydrogen" (an extra-reactive form of hydrogen - monatomic? electronically excited diatomic? both? something else?...) has been postulated and discussed for well over a century. Based on the current evidence, what is actually thought to happen in these cases is that the dissolving metal gives up electrons to the surrounding medium. Some of these get captured by DHH molecules to form a radical anion, which rapidly captures a proton, H⊕, to complete the first phase of the reduction (and which incidentally, illustrates the equivalence of gaining an electron and adding a neutral atom of hydrogen).
"But that's only one atom of hydrogen!" I hear you say. Well - you're quite right, the process gets repeated - accepting an electron to form a radical anion, followed by protonation. Then we have formed our THH from DHH. In the case of harmine - plain ol' H (careful!) - a grand total of, of course,
four electrons must be added to the molecule, along with the corresponding
four protons which jump aboard -
TetraHydroHarmine. THH.
“There is a way of manipulating matter and energy so as to produce what modern scientists call 'a field of force'. The field acts on the observer and puts him in a privileged position vis-à-vis the universe. From this position he has access to the realities which are ordinarily hidden from us by time and space, matter and energy. This is what we call the Great Work."
― Jacques Bergier, quoting Fulcanelli