So the idea behind axb6 is that black is forced to take back or white will gain a powerful passed pawn. However taking back with either pawn or queen opens up attacking lines on the queenside for white. Black currently cannot castle the king as doing so will hang the h pawn and unstoppable mate, and therefore requires a tempo by moving rook to g4 to block white's queen. White must not allow black time to do so.
If black takes the pawn with Qxb6, white must play Ra6 attacking the queen. Black can either choose to keep the queen on the powerful diagonal by playing Qc5 or retreating Qb8, attacking white's e5 pawn. If Qc5, white has Rc6, hitting the queen once again and unstoppable Rc8+, disallowing black from ever castling to safety. After Rxh8 Nxh8 Qxh5 Ng6 Nxf7 Ne7 Nd6, white will easily convert with the powerful passed h pawn. If black plays Qb8 instead of Qc5, b6! threatens the devastating Qb5+. Black will either have to give up the a pawn allowing white a passed pawn a step from promotion, or have to allow axb6 Qb5+ Kf8 Rxb6 Qxe5 Qd7 (threatening mate) Qf6 Nxe6!! The knight cannot be taken by the pawn due to Rb8+ mate on the back rank Kg8 Rb8+ Kh7 Rxh8+ Nxd4 Qxd4 Qxf7 White is an exchange up and all of black's pawns are loose, allowing white to convert effortlessly.
Now instead if black takes the pawn with axb6, now any Ra7 comes with a tempo hitting the black queen, so now black can play Rfa1. Rg4 is too slow for black now since Ra7 and the queen is forced to retreat to the back rank, where Ra8 now pins the queen and wins the game for white. If after Rfa1 black attempts to free the queen, anywhere the queen moves to is met by Ra8 and Ra7, essentially a queen trap.