Fear is one of the biggest handicap on downhills. Fear of falling and hurting yourself possibly badly. Confidence in running fast downhills comes with skill at running fast downhills. Practice will add to both.
Often where i run the downhill is wet, muddy and slippery so the butt technique is part of the course, but usually unintentional. My other favourite is the drunken dodo bird as my arms flap all over the place in anticipation of take-off
Adelaide is mostly flat unless you are up in the surrounding hills but there are a few spots with some declines to practice on and as Ken says once you learn to relax and not stiffen up, go rag doll floppy and stay low to the ground it is easy.
I enjoy the small dips that I can sail down, it makes a bit of interest from the same old horizontal.