"Available 24/7, it will be a weighted index based on some of the most popular bitcoin exchanges: Bitfinex, OKCoin USD, and Bitstamp — though no one exchange may have a weight over 50%.
To keep traders as well-informed as possible, the index price is updated every five seconds.
Kaiko will re-calculate the weights on the last Friday of every month, with data based on the previous 30 days’ volumes...
If a single exchange’s API goes down it is not de-weighted, but the price and weight are held constant until the next update. The weights are fixed, so users always know what is in the index — and it is very clear how API outages are handled."
Here's how its historical weighting looks between the three exchanges:
There's still a "fair basis" applied to the Index value to reflect the market and orderbook depth on BitMEX contracts, this has not changed.
The change will be live end of April.