It seems that the futures market for the nikkei mini starts at the same hour as their stock market. So when is the price discovery initiated to justify the gap ups and gap downs on the nikkei? There is no premarket for the nikkei futures?
In addition to what Kiwi mentioned, orders are accepted from 08:00 and the indications shown are usually quite close to what the opening will be. A major exception to this would be surprise economic data that comes out at 08:50 which is the scheduled time for such pre-opening announcements.