White could not prevent Black from recycling pieces; no matter what he did, there would always have been a row to capture either on the c7-i3 line, or on the g1-g7 line.
By playing 1. i4-f4, White lined up the two Black GIPF-pieces on the a5-i1 diagonal. He knows that Black must play 1. … i5-e5;x(Gg3), meaning that a white GIPF is pushed onto e5.  White then plays 2. a3-c5 and establishes an attack on Black’s Gf4 (and Gg3 if is still there) which cannot be neutralized.