Can you explain how it works and how to use it and what is the advantage compared with regular steep turns?
He needs to turn and start to descent. Note, that the two moments he's overbanking he's flying over ridges and wants to descent to the valley behind.
Now when you want to lower the nose in a turn you need to bank more so you still have the desired lateral lift, but less vertical.
But he's so fast that he needs negative vertical lift component, and since the wings are producing positive lift to make the turn, he needs to bank more than 90° to get it.
I have CPL but I was never thought such maneuvering. Can it be used in SEP aircraft or glider with just a couple of G?
It is a positive G manoeuvre and you can do the same with, say, 2G, so technically yes. However:
You have no use for it. He's flying 420–480 knots. At 420 knots, a 1G (meaning 1G lateral, so 1.41G total at 45° bank) turn will have rate 2.6°/s and radius 4759 m. But in a SEP you might be flying, say, 120 knots and then 1G turn will have rate 9.1°/s and radius 388 m. That, of course, applies to vertical turns as well. So where he needs to push -1G, and thus overbank, you would do with reducing to, say, 0.5G, and thus still normal side up.
You would probably loose spatial awareness rather quickly if you are not trained in aerobatics. The fighter pilots train a lot to be able to handle the plane at these high Gs and unusual attitudes. Without training, it will get confusing pretty quickly.
Your attitude indicator will likely tumble. The fighters have attitude indicators carefully designed not to, but your average mechanical one in something like C172 almost surely will. That is not a problem in itself—aerobatic pilots just cage it and fly purely on external references—but you need to be aware of it.
The fighter has a G-meter, but in a typical SEP plane you don't. But if you fly below $V_A$, the wing will stall before exceeding the structural limit, so you can do without.
During aerobatics, such, or similar manoeuvres may be performed. For example in Red Bull Air Race turns are usually executed at 90° bank (to use all the lift for the change of direction). The plane climbs a bit to store some speed in height, since turns are tighter at slower speed, then makes the hard turn, during which it has 0G vertical and the vertical speed decays and it changes from climb to descent and then it levels from the descent in the gate height and with all its speed back.
At the end of the movie looks like 5G is used for slowing airplane for landing. 5G is pulled and jet instantly slows from 300 to 260.
The induced drag is proportional to coefficient of lift. So when the wings are requested to generate a lot of lift, they also generate a lot of drag. Especially since this might have been enough to pull the delta wings of the Typhoon to the vortex lift regime, where it generates even more drag.
I believe part of the reason to fly such sharp turn here is to fly the pattern at modest size despite the high speed and the deceleration is just a nice side-effect. The speed brake would be enough to slow down, and still needs to be used as 260 knots is still about twice the landing speed.
