HOOVERS
those lawnmower/chainsaw/jet ski sounding slides.
Judging strictly by U.K. Hard House / Hard Trance / Hard Dance / NuNrg / whatever you want to call it. . .the formula appears quite simple :
This may sound cynical, but. . .
These types of tracks spend maybe 4 phrases (32 measures) in the intro w/ pulsing bassline (NuNrg has a more interesting bassline, f/ what I've heard), leads, angry synths, and then quickly a breakdown, leading to a 4 phrase build, with vocal cuts, hoovers, increasing occurance of sounds/loops, doubling up of loops ("doubling up" imm, means taking a 4 beat loop and just looping beats 1 and 2 over and over), a loud siren sounding thing, and then a measure before the peak, everything cuts out except for some background high synth, add vocal cut, drop in the track with a loud explosion sound and continue at this pace while gradually fading out individual tracks until the kick and snare at the end.
Some of it sounds good, most of it (that I've heard) sounds templatey. The variation comes from the different rythms, patterns, and melodies that are constructed by a combination of all the synths/hoovers/ etc.
There is not really that much which is "essential" for HH, unless you want to make one of those template tracks.
anyway, good luck bruddah