Actually down from the hand shaking position. That can be a steep angle but the disc should be at least parallel but lower to the largest bones in the forearm.
Yes i meant thumb outermost section being almost vertical pushing down on the flight plate. So that the rest of the thumb is not in contact with the disc and the pressure of the thumb pinch is not divided over as large of an area as with all of the thumb on the flight plate. More pressure=more friction=stronger grip. The flight plate will probably bend down some and add some more grip because the tip of the thumb needs to climb out of the pit uphill. In 2003 worlds video Climo showed regular thumb orientation intead of the Jenkins grip described above and called pushing the flight plate down as having control. It is easier to avoid slipping of the disc when the base of the thumb rests on the flight plate too and the tip to finger print corner of the thumb digs into the flight plate.
The further out toward the outer edge of the disc the thumb lies the more it points forward and can be ahead of the index finger. Since the thumb is on top of the disc pushing down on it will automatically drop the front of the disc and raise the rear as long as there is room for the disc to move into the deeper front down orientation. Which is not true for every grip. Regular power grip has the disc in the crease of the hand and the base of the thumb can be in the way. So it is a trade off in grip strength if you decide to raise the rear of the disc the lie near or at the innermost joint of the thumb. Because then the base of the thumb cannot push down on the flight plate.
The Wiggins grip does not have the index finger under the disc at all so the thumb is well ahead of the middle finger which is the foremost finger under the disc. It has the the thumb and the base of the thumb on the disc but from the way others described the grip you could get an even stronger grip by tilting the head of the thumb down like the Jenkins family.
I have not experimented with actual throws yet how extreme nose down angles you could get if you weakened the grip on top of the disc by removing the base of the thumb from the top of the flight plate. So that the rear of the disc can rise close to the innermost joint of the thumb. If you don't orient the middle and ring fingers upright on the rim but almost along the rim removing the pinky from under the disc you can get a fairly strong grip and pushing the wrist down when not throwing produced about 30 degree nose down angle on the disc when the hyzer angle was horizontal. Naturally doing the same while throwing is way harder. The wrist moves and if you don't use the hyper spin technique where the wrist does not bend to the left of resting position during the throw (or shouldn't turn) the bones n the wrist will try to push the hand up as it moves from bent back to the resting position. You can fight this but it takes some muscle power and some powerful guys here got that to work on the first day and weak old me am very inconsistent with how well i do. At my feeble strength i need to use so much of my muscle power in the arm to wrestle the hand to be down when the disc rips that the forearm muscles are so tense that it slows down the throw costing distance.
The solution is twofold. Have enough muscle power in the forearm to be able to stay loose enough in the muscles for good acceleration of the arm and cheating the wrist bones. Which can happen by not allowing the wrist to bend to the left and snapping from the resting position to the right (hyper spin technique) with the hand or allowing the wrist to bend a little snapping to straight or a little right of the resting position but keeping the hand at resting position in the vertical axis until the hand is coming straight in the snap in the horizontal axis and then pushing down. The latter version is insanely difficult to time and the order to bend the wrist down has to leave the brain early because that part of the wrist snap is probably under 1/100 seconds long. And the rip occurs in super quick succession after the disc pivots very quickly. And you need to pinch the disc too at that time and aim the disc and so on so there is more to do than the brain can handle so it needs to become automated with probably thousands of repetitions. After/if you get it right which may take a lot of throws and time. So do you have years to pursue that goal or are you one of the gifted ones that get it in the first attempt and can reproduce it always? Nobody knows before you try.
These are by no means the only grip variations that are out there and at least the Bonopane grip too can produce extreme nose down angles. It just tends to hurt in the fingers while throwing. I've not used it enough to see if the fingers would adapt to it and become pain free. There may be some other grip tricks that can help too but you've got a lot of experimenting to do with the above tricks already.

