G91               ; relative positioning
G1 H1 Y-400 F1800 ; move up to 400mm in the -Y direction, stopping if the homing switch is triggered (first pass)
G1 H2 Y5 F6000    ; go back a few mm
G1 H1 Y-400 F360  ; move slowly to Y axis endstop once more (second pass)
G90               ; absolute positioning