If the Archduke Charles's brother, John had been able to reach him with Austrian reinforcements, then the French would have been encircled and lost the battle, perhaps even bringing an end to the Napoleonic Wars as Napoleon would likely have been captured.
Had it happened I wonder how Austria would have made use of it. Would there have been the wars in Italy against Napoleon III. Could Prussia have managed 1866? Would Austria have ended up as the rump state it became after World War One?