I am glad the kite scares you a bit; that will help keep you safe.
A 4.7 m Fixed bridle (FB) is a good background for your first de-power as long as you have some good experience with it in a variety of winds. Then
the de-power kite will be a bit less intimidating.
I do not know the NewTech kites well but here is what I bet is going on: The kite is likely 2 line biased and flies well on the front 2- lines, so
loosening the brakes will speed it up. Tightening the brakes too much will choke the kite a bit, muting the lift and holding it back rather than
increasing lift or power.
[Soliloquy: Some FB kites, especially buggy engines and certain related race kites will benefit from the right amount of brake tension and actually
cause a surge in power, but that is another thread ]
Now your de-power is a different animal. You have a pulley system in the bridle that allows the entire kite to rotate and change the angle of attack,
rather than just deflecting the trailing edge. Thus when you pull back on the bar (rear lines), the kite increases angle of attack and powers up, and
when you sheet out and let the kite rotate forward and lower the angle of attack, you reduce lift and power, right? Well...
There is a phenomenon with de-power and newbies that messes up a lot of riders' heads. De-power kites need to be in constant motion (in a buggy, on a
board or skis, whatever) in order to behave as advertised. But newbies wisely start flying de-power static and therein lies the rub. When you fly
static, the kite is not in motion long as it passes briefly through the window to the other edge. Thus you will see behavior much like your FB
NewTech. Pull on the bar and the kite chokes and stalls instead of powering up. Let the bar out and maybe trim in the center lines (for full
de-power, right?) and the kite speeds up and actually generates quite a lot more power! Backwards!
When you and the kite get in motion, you will see the kite behaving as you would expect (or have read), and then you will learn to trim the kite for
the conditions and the power that you want or need. Treat the center strap as your transmission (stick shift) and the bar as your throttle. If you
start out with the center strap too far out (low gear) you will stall the kite (engine) when you hit the throttle (pulling the bar). Start with your
center strap pulled in (low gear) and tool around.