Both my training partner and I felt sitting on your opponent from high mount ala Roger Gracie is more effective than hips up because it creates more pressure on the bottom player, making it difficult to breathe:
To counter being rocked forward, we centered our head over hips, which likewise adds centrifugal weight. If our opponent does bridge, we surfed with our butts, raising hips just enough to nullify bottom players bridge. Once their hips went down, ours went down, putting pressure back on (probably a principle here you could call ‘shadowing’ or something?). Regardless, the position is very loose, as dead body weight is heaviest!
For those of you who have Saulo Ribeiro’s Jiu-Jitsu Revolution 1 DVD series, Saulo demonstrates hip lift described above there. Looked for a YouTube video, but couldn’t find one! To reiterate, I’m lifting my hips like Henry Akins below, but only as much as I need to, before sitting back down in Roger’s posture. Bottom player is carrying appoximately 70% of my weight on their chest through my bum. Not fun, but reminds training partners why Mount Bottom is not a happy place structurally. ;o)