unique magical effect
Extremely powerful jet of golden fire between two wands.
References from the canon
Harry's wand spun and shot golden fire at Voldemort on its own (DH4). This effect was generated by the wand itself using power gained from interaction with its "twin" wand.
The golden fire produced is possibly connected to the phoenix feather inside each wand. This is implied in the text but not stated as fact, so therefore not canon.