Silky,
"the transfer valves automatically open when the inner tank reaches the low level (about 750 kg)"
"Note: The 750 kg value is based on a level aircraft attitude with no acceleration. During steep descents or accelerations/decelerations, the transfer tank valves may open with more than 750 kgs of fuel in each inner tank and the low-level warning may be triggered"
Courtesy of A320 Vol 1.
I guess that would include pitching down for descent.