A couple of years ago Ernst said he planned to give a course on the extended use of underlying/overlying cards. Since then he has presented his impressive course on PAC in astrology and is about to reintroduce the Aditya zodiac, so I don´t suggest that he should prioritze the cards right now.
With that said, perhaps we could come together in the community and try to find out what it´s all about?
My suggestion:
1. The underlying/overlying card gives a direction to each card. The energy in the card doesn´t just work in a certain way, it does it for a deeper reason. It wants to develop in a certain direction. We are here for to learn. It´s about maturation.
So, for example, the Mercury card in my birth spread, 5C, is 7D. It´s underlying and overlying card is 9H, which is a fool card. It´s all about connection to divine love through surrender. So there is a deep defaitism in the psyche of a /healthy/ 5C, using desilusions and "bad" experiences as reason to give up to the divine. A shrimp mentality in a way - wherever the current may take us... As underlying card 9H says: "I can do this". It´s familiar. As overlying card it says: "I have to do this! It´s my direction." (And I find it´s fascinating in that context that the Jupiter card is the protective 5S, helping us to avoid many of the most dangerous challenges, and that we have Kings on the EC and Uranus card to motivate us to accept sacrifices. While the Sun card 3D is good at saying "no" to things and the Moon card AS gives the strength to start over after a crisis and reveals secrets.)
That would be one way of using the U/O cards - to get a feeling of what the card is about in a deeper level.
2. And then, of course, we should look for appearances in the temporal spreads of the birth spreads U/O cards. So that, using the example above, any time a 9H shows up in the progressed spread or a year spread for any card that has a 7D in it, it means that the 7D experience is deepening, getting more mature.
Those are more or less my thoughts about this theme so far.
Any more thoughts, anyone?
Not asking you Ernst, if your read this, you focus on the Adityas, please! 
Stafffan
Oh, one more thing: The second method should specially helpful when we want to assess anything related to blocked cards. If the card holds a separating factor, the extra push of having the same card as the underlying/overlying card of that card could be the help that it needs to start working better.
Staffan
Yet another thing is that it helps us to understanding the dynamics between the cards.
An example from my own chart:
My (5C) Mars card is 7H. It´s underlying card is 8H, self love, being emotionally balanced. So self love and emotional balance is the foundation for the capacity that a non obstructed 7H Mars card gives to a 5C for to overcome losses.
The Ecliptica card, KH, tells us that life will bring 5C events that force them to have this experience. The overlying card - the "outcome" to a degree - of 7H is AS, which happens to be the Moon card.
So disappointments in relationships and experiences of "tough love" (KH), teach 5C (Mars being the school, the training card) to let go and accept losses, which takes them directly to the AS Moon card: starting all over and discover new things about themselves.
The outcome of the Moon card is exactly that: 2C (The overlying card of AS), so new knowledge, new insights.
That same 2C happens to be the foundation, through quadration this time, to the overlying card of the Birth Card, which is 4C, in a way expressing the goal of the whole spread.
From the Ecliptica card to the overlying card of the BC there is an unbroken thread, through the underlying and overlying cards.
AND the Moon card´s overlying card - 2C - is also the underlying card of the Ecliptica card KH. So around it goes...
The overlying card of the Ecliptica card is 9S, which is also the Rahu card..
Staffan
@mitryendra80 I wonder whether you have any thoughts on this? I have this idea that it might be interesting to make a couple of videos on this for that page that Mirela says she has, to which she invites people to make videos. In case she´s interested. What about the two of us having a discussion about it on Zoom and publish the recording?
Or, wait, is it amit that I should ask? I don´t remember right now, are you into the cards?
Just a thought.
Staffan
@staffan Thanks for the invitation but I haven't taken a real dive into the cards yet.
It's a bit digital, or, you know, "black and white" for me. It also raises some astronomical questions.
I would, however, be interested in the theory behind overlaying and underlying cards. It makes me think of the from above/from below of the nakshatra sutras.
@mitryendra80 Ok, well, I was confused and should have asked Amit, of course. Which I will do.
And yes, with the underlying/overlying cards combined with the numerological indications, you can really follow the dynamics and how the energy flows; one energy leading over to another. I mean, numerologically a two is the basis for a four (square) and a three for a nine, etc.
"Morphing" is the word that comes to me.
Interesting what you say about the nakshatras, I haven´t grasped that concept yet. The gunas are also very abstract to me when it comes to the nakshatras.
In Maya tradition there is a calendar called "Cholqij" that I´m quite familiar with; there are twenty energies that works on 13 different energy levels, together forming a calendar of 260 days, more or less the same time as a child to grow in it´s mothers womb and the time from planting and harvesting the maiz. In that calendar each energy develops in a similar way, so that my "nahual" - that´s what the energies are called, it´s the "sign" - "Imox" (which is water), develops out of "Aj" (the maiz stalk, representing the clan and the family) and growing into a "Toj" experience (the offering). It´s also supported by an extra energy on each side, the spiritual and the practical. And it really describes who you are. I´m very much rooted in my ancestry, family always comes first, and I´m very much into offering ceremonies; I´m also a sundancer in which tradition the offering/sacrifice is a central concept.
The concepts develop over time, so that the "root" represents the day of conception and the "destiny" is what you grow into; after 52 you more or less become that other energy, exactly as in the Cards of Truth in which you more or less become your overlying card after 52.
Staffan