The rate takes all qualifying trades between 15:00 and 16:00 London time, splits the hour into twelve partitions, and uses a volume-weighted median of each partition before averaging them. The partitioning and median are there specifically to make the benchmark expensive to manipulate.
Because the whole settlement rests on one hour of spot trading, that window sees a volume spike, and futures can dislocate from spot elsewhere in the day without affecting where contracts actually settle.
Example: spot bitcoin trades between $59,400 and $60,900 during the window with most volume near $60,100. The BRR prints $60,140, and every expiring contract settles there regardless of the $61,500 print an hour later.
Related: bitcoin-futures, ether-futures, final-settlement, cash-settlement, special-opening-quotation