The Hippopotamus opening i know is based on moves a6 b6 d6 e6 g6 h6 without! moving c and f pawn in opening.U pieces then normally go bg7 bb7 and knights e and d7.The Point is u reach a very flexibel pawn stuktur in opening,reason is no piece blocks any pawn like a nf3 blocks often f pawn.Ideal would further then if u reach a position like in kingsindian or pirc,where after d4d5 by white and e6e5 by black the best way for black would be 2 play f7f5,in that case u dont need to regroup u pieces and a able 2 play f7f5 at once.Also Gm Petrosian used this opening in fight for worldchesschampionship i think 1966 or 1969.But yes,aynway which opening u play,where there is light there is always also shadow;-)
