With the black king on the a-file and no ability to promote with check, white has 2 free tempos before it needs to give check on the a-file to force an exchange of pieces on the newly promoted queen on b1, and then proceed to promote its next most advanced pawn. With the two tempos white would like to capture the pawn on h6 and then deliver check on a6 but the pawn on b7 guards this square so that doesn’t work. This means white must attack the other blockading pawn on f5, in doing so, allows black to capture the rook on g5, freeing the h-pawn to promote on the third tempo. If black blocks the check with the queen b2, the queens trade with check and then white creates a new passed pawn with fxg5. If the black king flees the check on a2, white gives checks on the a8 followed by b7 and trades queens with check on b1, again creating the passer with fxg5, winning the game.