Mantis pioneered the "composite" construction by using the oversized aluminum front triangle and combining it with the smaller diameter cro-moly rear end.  The idea was to give the rigidity and efficiency of the big aluminum tubes but combine it with the suppleness of the smaller tubing in the rear end.  The small stay also allowed for better tire clearances.  The elevated chain stays eliminated the chain suck issue and allowed chain maintenance without a chain tool but ultimately died due to the excess weight and complexity of the extra tubing plus difficulties in reducing bottom bracket flex.  The composite construction was also found in the Fisher CR-7 plus the Cunningham designed Nishiki Alien series.  Elevated chain stays were found on many bikes of the era including the Yeti Ultimate, Nishiki Alien, Brave Warrior plus others.