The spell Flaywind Burst (from Sandstorm) says the following:
This spell produces a brief windstorm (approximately 70 mph), filled with scouring, supernatural grit that literally strips flesh [...] A creature within the area of flaywind burst must make a Fortitude save or experience the effects of the wind's force [...] Large or Huge creatures are unable to move forward against the force of the blast, or if flying are blown back 1d6×5 feet.
Since the spell is Instantaneous yet has a full round casting time, how long & when does the wind affect the creatures of size large? Would they be unaffected on their turn, the spell essentially doing nothing but damage vs. large or larger creatures, or does it affect them in some way?
Casting Time: 1 round
Range: 60 ft.
Area: Cone-shaped burst