hehe... Yeah very confusing. "hypo" actually refers to plagal or under. This will help separate the differences between "hypo" modes and authentic modes, the modes that we see and use today. There is an application for these "hypo" modes but it may only be for your own understanding.

Take a simple phyrgian mode: E, F, G, A, B, C, D, E. It's relative major scale would start with C, this we will call a reciting tone. i.e. C major has the same notes as E phrygian. Now to say something is written in a mode (in in a diatonic scale), what has to be true?? The song or melody will have to end of in tonal (modal if you wish) note, which in this phrygian scale is E. This how the mode is looked at today because it is more acceptable and modern sounding.

Now for a hypophrygian mode: B C D E F G A B. Which is a fourth under the phrygian mode above. A perfect forth below E is B, so that's where this scale starts from. This is why this mode is plagal. i.e. if you know your cadances IV-I is called a plagal cadence, same thing. Now what is the fourth note in the hypophrygain, it's E. A perfect fourth above B, so in this scale (like the phrygain) E will be the tonal note. In a melody or whatever E will still be the final note played, making it sound plagal. i.e since this is a B locrian mode, if you were to play this scale over a Bm7b5 chord and you ended the melody on E. You will have a plagal sound, or hypophrygain. Back to the first which is E phyrgian scale so it would work over a E minor chord, but you would end of E.

Another thing is in the hypophrygian mode the reciting tone is A instead of C, which is the relative minor of C major. The thing that contrasts these two modes is the hypophrygain will sound more minor than the phrygain. Ending on the fourth tone of a chord your playing over will have more of a minor sound, “plagal“ in truth.

This almost seems useless, but if any reason you want to write a tune that sounds pre-sixteenth century, this is a good thing to know. Alot of tunes from this time ended on the IV chord instead of the I chord. Hopefully that wasn’t to confusing. I find that if you begin a melody or whatever with the reciting tone then the results are better. i.e. writing a tune in the phrygain mode will often begin with it’s relative major C, but will end on E. Instead of ending with the tonic. Which is what sounds modal. In the case of hypophyrgian, it sounds more apparent if you begin with the relative minor when ending on the phrygian tone of a major scale.

Hopefully that makes sense.